🎯⭐ INTERACTIVE LESSON

Spectroscopy

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Spectroscopy - Complete Interactive Lesson

Part 1: Functional Groups & Nomenclature

Organic Chemistry for the MCAT

Part 1 of 7 — Functional Groups & Stereochemistry

Must-Know Functional Groups

Group	Structure	Example
Alcohol	$-OH$	Ethanol
Aldehyde	$-CHO$	Formaldehyde
Ketone	$-CO-$ (internal)	Acetone
Carboxylic acid	$-COOH$	Acetic acid
Ester	$-COOR$	Ethyl acetate
Amide	$-CONHR$	Peptide bond!
Amine	$-NH_2$	Methylamine
Ether	$-O-$	Diethyl ether

Stereochemistry

Chirality: 4 different groups on a carbon → chiral center
Enantiomers: Non-superimposable mirror images (same physical properties except optical rotation)
Diastereomers: Stereoisomers that are NOT mirror images (different physical properties)
Meso compounds: Have chiral centers but an internal plane of symmetry → optically inactive

R/S Assignment (Cahn-Ingold-Prelog)

Assign priority by atomic number (highest = 1)
Orient lowest priority group away from you
1→2→3 clockwise = R; counterclockwise = S

Stereochemical Relationships You Must Distinguish

Constitutional isomers: same formula, different connectivity
Stereoisomers: same connectivity, different 3D arrangement
Conformational isomers: interconvert by bond rotation (usually not isolated)

On the MCAT, many questions hide stereochemistry inside a passage about receptor binding where only one stereoisomer is biologically active.

Functional Groups & Stereochem 🎯

Key Takeaways — Part 1

Know ALL functional groups instantly — they appear in every MCAT passage
$2^n$ rule for maximum stereoisomers
Enantiomers: mirror images, same properties (except rotation). Diastereomers: different properties.
Amide = peptide bond — this connects to biochemistry
Always classify relationship first: constitutional vs stereoisomer vs conformer.

Worked Examples — Functional Groups & Stereochemistry

<details> <summary>Example 1: Assign R/S configuration to a chiral center</summary>

Question: Assign R or S to this chiral carbon:

Where: 1=Cl, 2= $CH_{3}$ , 3=OH, 4=H

Solution:

Assign atomic numbers: Cl (17) > OH (8) > $CH_{3}$ (6) > H (1)

</details> <details> <summary>Example 2: Count stereoisomers using the $2^{n}$ rule</summary> <pre> CHO | CHOH ← chiral center 1 | CHOH ← chiral center 2 | $CH_{2}OH$ ← NOT chiral (two H atoms) </pre> </details> <details> <summary>Example 3: Distinguish stereoisomer relationships</summary> </details> <details> <summary>Example 4: Identify a meso compound</summary> <pre> H | Br-C-H | $CH_{3}-C-CH_{3}$ | Br </pre> </details>

Part 2: Stereochemistry

Organic Chemistry for the MCAT

Part 2 of 7 — SN1, SN2, E1, E2 Reactions

Substitution vs. Elimination Decision Tree

Factor	SN2	SN1	E2	E1
Substrate	Methyl/1°	3°	3° (or 2°)	3°
Nucleophile	Strong	Weak	Strong BASE	Weak base
Solvent	Polar aprotic	Polar protic	—	Polar protic
Mechanism	1 step, backside	2 steps, carbocation	1 step, anti	2 steps
Stereochem	Inversion	Racemization	Anti-periplanar	—

Key Points

SN2: Rate = . Backside attack → inversion. Sterically hindered substrates slow it.

Part 3: Substitution & Elimination

Organic Chemistry for the MCAT

Part 3 of 7 — Carbonyl Chemistry

Carbonyl Reactivity

The $C=O$ is polar: carbon is electrophilic (attacked by nucleophiles).

Aldol Condensation

$\text{Enolate} + \text{Aldehyde} \to \beta\text{-hydroxy carbonyl} \xrightarrow{\text{heat}} \alpha,\beta\text{-unsaturated carbonyl}$

Part 4: Carbonyl Chemistry

Organic Chemistry for the MCAT

Part 4 of 7 — Carboxylic Acid Derivatives

Reactivity Order (most reactive → least)

$\text{Acid halide} > \text{Anhydride} > \text{Ester} > \text{Amide} > \text{Carboxylate}$

Why? The better the leaving group, the more reactive.

Acid halide: Cl $^-$ is excellent leaving group

Part 5: Carboxylic Acid Derivatives

Organic Chemistry for the MCAT

Part 5 of 7 — Aromatic Chemistry & Lab Techniques

Aromaticity Rules (Huckel)

Must have: planar ring, conjugated $\pi$ system, $4n + 2$ $\pi$ electrons ( $n = 0, 1, 2...$ )

Part 6: Spectroscopy & Structure

Organic Chemistry for the MCAT

Part 6 of 7 — Spectroscopy (NMR, IR, Mass Spec)

IR Spectroscopy — Key Absorptions

Bond	Wavenumber (cm $^{-1}$ )	Shape
O-H (alcohol)	3200-3600	Broad
O-H (carboxylic acid)	2500-3300	Very broad
N-H	3300-3500	Medium
C=O	1700-1750	Strong, sharp
C-O	1000-1300	—

H NMR — Quick Guide

Part 7: Review & MCAT Practice

Organic Chemistry for the MCAT

Part 7 of 7 — Review & MCAT Strategy

Highest-Yield MCAT Organic Topics

SN1/SN2/E1/E2 — almost guaranteed
Functional group recognition — in every passage
Amino acid chemistry — bridges to biochemistry
Carbonyl chemistry — reduction/oxidation
Stereochemistry — R/S, enantiomers vs diastereomers
Lab techniques — separation and purification

Amino Acid Side Chain Chemistry (bridges to Biochem)

Property	Amino acids
Nonpolar	Gly, Ala, Val, Leu, Ile, Pro, Phe, Trp, Met
Polar uncharged	Ser, Thr, Cys, Tyr, Asn, Gln
Positive (basic)	Lys, Arg, His
Negative (acidic)	Asp, Glu

Test-Day Organic Workflow

Identify the functional group(s) first.
Decide mechanism class (substitution, elimination, addition, acyl substitution).
Check stereochemical consequence (inversion, racemization, retention).
Use reagent strength and solvent to resolve competing pathways.

Worked Example — Predicting an Amino Acid's Charge at a Given pH

Scenario: A passage asks for the net charge on the side chain of lysine (side-chain p) in blood at , and how that compares with (side-chain p).

Priority 1 = Cl
Priority 2 = OH
Priority 3 = $CH_{3}$
Priority 4 = H

Orient H (priority 4) away from viewer ✓ (already shown)

Trace 1→2→3:

1 (Cl) → 2 (OH) → 3 $(CH_{3})$
This traces counterclockwise = S-configuration

MCAT Strategy: Draw/mentally rotate to get H in back. Then trace 1→2→3 path. Clockwise=R, counterclockwise=S. Practice this 10× before test day.

Question: How many stereoisomers exist for 2,3,4-trihydroxybutanal?

Identify chiral centers: C2 and C3 have 4 different groups each → 2 chiral centers
Maximum stereoisomers = $2^n = 2^2 = 4$
These are: 2R,3R / 2R,3S / 2S,3R / 2S,3S

MCAT Strategy: $2^n$ rule doesn't account for meso compounds (which reduce the number). For this molecule, check if any stereoisomer has an internal plane of symmetry. (It doesn't, so answer = 4.)

Question: Molecules A and B both have formula $C_{4}H_{8}Cl_{2}$ . A has both Cl atoms on C1 (geminal), and B has Cl atoms on C1 and C2. What is their relationship?

Connectivity differs: A = 1,1-dichlorobutane; B = 1,2-dichlorobutane
Different connectivity means constitutional isomers (not stereoisomers)
They are NOT related by stereochemistry alone

MCAT Strategy: If the atoms are in different positions (different connectivity), don't even look at stereochemistry—they're constitutional isomers.

If they had same connectivity, different 3D arrangement? Then determine stereoisomer relationship: enantiomers (mirror images) or diastereomers (not mirror images).

Question: Is this molecule chiral?

(Assuming this is drawn with internal plane symmetry)

It has 2 chiral centers (both carbons have 4 different groups)
BUT if you draw it in 3D, the molecule has an internal plane of symmetry
The left half is the mirror image of the right half
Result: Chiral centers exist, but molecule is achiral overall (meso compound)
The molecule does NOT rotate plane-polarized light

MCAT Strategy: Meso compounds are rare on the MCAT, but they're a "gotcha." Always ask: "Does this have a plane of symmetry?" If yes, it's achiral despite chiral centers.

k [substrate] [nuc]

SN1: Rate =

k[\text{substrate}]

. Carbocation intermediate → racemization. Favored by 3° substrates, polar protic solvents.

E2: Strong BULKY base (t-BuOK) favors elimination over substitution. Anti-periplanar geometry required.

E1: Shares carbocation intermediate with SN1. Heat favors elimination.

Fast Test-Day Decision Rules

If you see methyl/1 degree + strong nucleophile + aprotic solvent, think SN2 first.
If you see 3 degree substrate + protic solvent, think SN1/E1 competition.
If you see strong bulky base + heat, think E2 (often Hofmann product favored).

Reaction Mechanisms 🎯

Key Takeaways — Part 2

SN2: strong nuc + methyl/1° + polar aprotic → inversion
SN1: weak nuc + 3° + polar protic → racemization
E2: strong bulky base + 2°/3° → Zaitsev product
The SN1/E1/SN2/E2 decision chart is GUARANTEED on the MCAT
Tie mechanism choice to substrate class first, then reagent/solvent/temperature.

Worked Examples — Substitution & Elimination

<details> <summary>Example 1: Determine mechanism from substrate + reagent</summary>

Question: Predict the main product(s) when (R)-2-bromopropane is treated with potassium ethoxide (KOEt) in ethanol at 75°C:

<pre> $CH_{3}$ | (R)-H-C-Br + KOEt (excess) → ? | $CH_{3}$ </pre>

Solution:

Substrate class: 2° (secondary) – can undergo SN2, E2, SN1, or E1
Nucleophile/Base: KOEt is a strong, bulky base
Solvent: Ethanol – polar protic but alkoxide is NOT solvated as well
Temperature: 75°C – high temperature favors elimination

Decision:

Strong bulky base + heat → E2 mechanism (elimination dominates)
SN2 is suppressed because KOEt is too bulky for backside attack
Product: Mainly 2-methylpropene (Hofmann/alkene product) with R/S stereochemistry lost (now a C=C double bond)

MCAT Strategy: The phrase "strong bulky base + heat" is shorthand for E2. Prioritize substrate class first, then base strength, bulk, and temperature.

</details> <details> <summary>Example 2: SN1 vs. E1 from a 3° carbocation</summary>

Question: 1-bromo-1-methylcyclohexane is dissolved in water at room temperature. What is the major product?

Solution:

Substrate class: 3° – strongly favors SN1/E1
Nucleophile: Water – weak nucleophile, weak base
Solvent: Water – polar protic (stabilizes carbocation)
Temperature: Room temperature – doesn't favor elimination overly

Decision:

Carbocation forms readily → SN1/E1 competition
Water attacks the carbocation → SN1 product (3° alcohol) dominates at room temp
E1 product (alkene) forms as minor by-product

Products: ~70% (1-methylcyclohexan-1-ol); ~30% (1-methylcyclohexene)

MCAT Strategy: At room temperature in water, nucleophilic attack dominates. Alcohol is major product. If the question said "heat," elimination would increase.

</details> <details> <summary>Example 3: Why SN2 fails on bulky substrates</summary>

Question: (1S,2S)-1-bromo-2-methylcyclohexane is treated with KCN (strong nucleophile) in DMSO. What happens?

<pre> |Br 4-step: / 1. Identify chiral center at C1 (2° carbon bearing Br) / 2. $CN^{-}$ is strong, DMSO is polar aprotic (ring) 3. BUT the cyclohexane ring creates steric hindrance 4. SN2 still wins: backside attack occurs </pre>

Solution:

Substrate: 2° but IN A RING with bulky neighbors (gem-dimethyl effects from fused rings, methyl group on C2)
Nucleophile: $CN^{-}$ – strong, small, polar aprotic solvent
Expected mechanism: SN2 (good nucleophile + aprotic solvent)

BUT: The ring + adjacent methyl makes backside attack difficult. SN2 is heavily retarded.

Minor: SN2 (nitrile product with config inversion, if it happens)
Major: E2 or SN1 (ring structure forces competing pathways)

MCAT Strategy: Even ideal SN2 conditions (strong nucleophile + aprotic) fail if substrate is too sterically hindered. Sometimes ring systems and bulky groups suppress SN2 entirely.

</details> <details> <summary>Example 4: Stereochemistry loss in SN1</summary>

Question: (R)-2-iodooctane in aqueous $AgNO_{3}$ (SN1 conditions) produces a 50:50 mixture of (R) and (S) alcohols. Why not 100% inversion or 100% retention?

Solution:

$Ag^{+}$ abstracts $I^{-}$ → carbocation forms
Carbocation is planar $(sp^{2} hybridized)$

Why not 100% one product?

The carbocation has no stereochemistry (planar, achiral)
Once it forms, the stereochemical information is lost
Product ratio depends on attack from both faces

MCAT Strategy: SN1 → racemization. Inversion suggests SN2. Retention is chemically rare (rearrangement-driven).

</details>

α,β-unsaturated carbonyl

Key Carbonyl Reactions

Reaction	Produces	Mechanism
Reduction of aldehyde	1° alcohol	NaBH $_4$ or LiAlH $_4$
Reduction of ketone	2° alcohol	NaBH $_4$ or LiAlH $_4$
Reduction of carboxylic acid	1° alcohol	LiAlH $_4$ only (stronger)
Oxidation of 1° alcohol	Aldehyde (PCC) or carboxylic acid (Jones)	Depends on reagent
Oxidation of 2° alcohol	Ketone	PCC, Jones, or K $_2$ Cr $_2$ O $_7$
Fischer esterification	Ester	Acid + Alcohol + H $^+$ catalyst

MCAT High Yield: Reducing Agents

NaBH $_4$ : mild, reduces aldehydes and ketones only
LiAlH $_4$ : strong, reduces ALL carbonyls (including esters, carboxylic acids, amides)

Nucleophilic Addition vs Acyl Substitution

Aldehydes/ketones: nucleophilic addition (no leaving group on carbonyl carbon)
Carboxylic acid derivatives: nucleophilic acyl substitution (tetrahedral intermediate + leaving group)

Recognizing whether a leaving group is present is often enough to choose the mechanism class.

Carbonyl Chemistry 🎯

Key Takeaways — Part 3

NaBH $_4$ : mild (aldehydes/ketones only). LiAlH $_4$ : strong (everything).
PCC: mild oxidation (1° ROH → aldehyde). Jones/CrO $_3$ : full oxidation.
Fischer esterification: carboxylic acid + alcohol + acid catalyst → ester + water
Amide bonds (peptides) are resistant to hydrolysis — that's why enzymes are needed!
Ask first: addition (aldehyde/ketone) or acyl substitution (derivative with leaving group)?

Worked Examples — Carbonyl Chemistry

<details> <summary>Example 1: Choose the right reducing agent</summary>

Question: You want to reduce ONLY the ketone in this compound without touching the ester:

Which reagent should you use?

Options: $NaBH_{4}$ / $LiAlH_{4}$ / $H_{2}SO_{4}$ / Zn/HCl

Solution:

Identify functional groups: ketone (left C=O) and ester (right C=O with $OCH_{3}$ )
Goal: Reduce only ketone to 2° alcohol
$NaBH_{4}$ : Reduces aldehydes & ketones → leaves esters alone ✓
: Reduces BOTH ketones AND esters → not selective

Answer: $NaBH_{4}$ (selective for ketone)

MCAT Strategy: Always memorize: $NaBH_{4}$ = selective for aldehydes/ketones. $LiAlH_{4}$ = no selectivity (overkill). This question pattern appears every 3-4 years on the MCAT.

</details> <details> <summary>Example 2: Oxidation level determines reagent choice</summary>

Question: Start with 1-pentanol. You want to make pentanal (not pentanoic acid). Which oxidizing agent is correct?

<pre> $CH_{3}CH_{2}CH_{2}CH_{2}-CH_{2}-OH$ → $CH_{3}CH_{2}CH_{2}CH_{2}-CHO$ 1-pentanol pentanal </pre>

Solution:

Target: 1° alcohol → aldehyde (oxidation level +1)
Wrong: $Jones/CrO_{3}$ (over-oxidizes to carboxylic acid)
Wrong: $KMnO_{4}$ (too strong, over-oxidizes)

Mechanism:

Mild oxidation (PCC) → Aldehyde
Strong oxidation (Jones) → Carboxylic acid

Product: Pentanal $(CH_{3}CH_{2}CH_{2}CH_{2}CHO)$ with no over-oxidation

MCAT Strategy: "1° alcohol needs to be an aldehyde" → PCC is your friend. Practice this memorization: 1° → aldehyde (PCC), 2° → ketone (any oxidizing agent), carboxylic acid (strong oxidation or Jones).

</details> <details> <summary>Example 3: Predict aldol condensation product</summary>

Question: What is the major product when acetaldehyde $(CH_{3}CHO)$ undergoes an aldol condensation followed by dehydration?

<pre> O O ‖ ‖ $CH_{3}-C-H$ (aldehyde with α-H's on the methyl) </pre>

Solution:

Enolate formation: α-H on acetaldehyde deprotonated by base → enolate ( $CH_{2}$ = $CHO^{-}$ )
Nucleophilic attack: Enolate attacks the carbonyl carbon of a second acetaldehyde molecule
Aldol adduct: <pre> $CH_{3}-CHOH-CH_{2}-CHO$ (3-hydroxybutanal / aldol adduct) </pre>
Dehydration (heat + acid): Water leaves from aldol adduct
Final product: , =CH-CHO (α,β-unsaturated aldehyde)

Key: The double bond forms between the α-carbon and the carbonyl-bearing carbon

MCAT Strategy: Aldol condensations create a new C-C bond and introduce an α,β-unsaturated carbonyl (which stabilizes via conjugation). The product is usually smaller molecules (acetaldehyde) condensing to form crotonaldehyde.

</details> <details> <summary>Example 4: Nucleophilic addition vs. acyl substitution mechanism choice</summary>

Question: Predict whether acetone $(CH_{3}COCH_{3})$ reacts via nucleophilic addition (NA) or nucleophilic acyl substitution (NAS) when treated with methylamine $(CH_{3}NH_{2})$ :

Solution:

Structure of acetone: $CH_3-C(=O)-CH_3$

Compare to acyl substitution:

<pre> If you had: $CH_{3}-C$(=O)$-OCH_{3}$ (methyl ester — HAS leaving group $OCH_{3}$) Then: Nucleophilic ACYL substitution occurs Product: $CH_{3}-C$(=O)$-NHCH_{3}$ (amide, with $OCH_{3}$ leaving as methoxide) </pre>

MCAT Strategy: Ketones/aldehydes → NA (no leaving group). Esters/acid halides/anhydrides → NAS (leaving group present). This is a fundamental distinction tested every year.

</details>

Amide: NH

_2^-

is terrible leaving group → most stable

Key Interconversions

Acid halide + ROH → Ester
Acid halide + RNH $_2$ → Amide (how peptide bonds form in lab!)
Ester + H $_2$ O (acid/base) → Carboxylic acid + ROH (hydrolysis)
Ester + NaOH → Carboxylate + ROH (saponification = soap making!)

Biochemistry Connection

Thioester (CoA derivatives) are key metabolic intermediates — more reactive than regular esters due to weak C-S bond.

Core Mechanistic Pattern

Most derivative reactions proceed through:

Nucleophilic attack on carbonyl carbon
Tetrahedral intermediate formation
Collapse and leaving-group departure

The best leaving group generally determines the direction and feasibility of interconversion.

Carboxylic Acid Derivatives 🎯

Key Takeaways — Part 4

Reactivity of acid derivatives: halide > anhydride > ester > amide
Saponification = base hydrolysis of an ester → soap (carboxylate salt)
Thioesters (e.g., acetyl-CoA) are biologically activated intermediates
Peptide bond = amide bond — resistant to hydrolysis (needs enzymes)
Reaction prediction improves if you compare leaving-group quality first.

Worked Examples — Carboxylic Acid Derivatives

<details> <summary>Example 1: Predict nucleophilic acyl substitution from reactivity ordering</summary>

Question: Compare the reactivity of these compounds toward nucleophilic attack:

<pre> (A) $CH_{3}-C$(=O)-Cl (Acid chloride) (B) $CH_{3}-C$(=O)-O-C(=O)$-CH_{3}$ (Acetic anhydride) (C) $CH_{3}-C$(=O)$-OCH_{3}$ (Methyl ester) (D) $CH_{3}-C$(=O)$-NH_{2}$ (Primary amide) </pre>

Solution:

Leaving group quality comparison:
- (A) $Cl^{-}$ = excellent leaving group → most reactive
- (B) $CH_{3}COO^{-}$ = good leaving group (stabilized by acetyl C=O) → 2nd most reactive
- (C) $CH_{3}O^{-}$ = moderate leaving group → less reactive than B
- (D) $NH_{2}^{-}$ = terrible leaving group (basic) → least reactive
Resonance effects (secondary factor):
- (D) $NH_{2}$ donates electron density to C=O, reducing electrophilicity
- (A) Cl is electron-withdrawing by induction, increasing C=O electrophilicity
Reactivity order: (A) > (B) > (C) > (D)

MCAT Strategy: If you forget the exact order, remember: the better the leaving group, the more reactive. Cl > O-C(=O) > O-alkyl > $NH_{2}$ .

</details> <details> <summary>Example 2: Ester to amide transformation via nucleophilic acyl substitution</summary>

Question: Methyl acetate $(CH_{3}COOCH_{3})$ reacts with excess aniline $(C_{6}H_{5}NH_{2})$ to form an amide. Show the product and explain why the ester is displaced:

Solution:

Nucleophilic acyl substitution mechanism:
- Aniline (nucleophile) attacks the ester carbonyl
- Tetrahedral intermediate forms
- Methoxide $(CH_{3}O^{-})$ is a weaker leaving group than aniline is as a nucleophile
- But we must compare: Which is the better leaving group?
Leaving group comparison:
- $C H_{3} O^{-}$ ≈ 15 (methanol) → decent leaving group

Biochemistry connection: Peptides are made this way in lab (activating carboxylic acids), and the resulting amide bonds resist hydrolysis because $NH_{2}^{-}$ is a terrible leaving group. This stability is why enzymes are required to break peptide bonds in the body.

MCAT Strategy: Ester → Amide is thermodynamically favorable because the resulting amide is so unreactive (stability of product).

</details> <details> <summary>Example 3: Saponification (base hydrolysis) of an ester</summary>

Question: A triglyceride (fat with 3 ester groups) is treated with excess NaOH in ethanol (soap-making process). What happens?

<pre> (Fat structure simplified as R-C(=O)$-O-CH_{2}-CH(OH)-CH_{2}-O-C$(=O)-R') (ester arms on triglyceride backbone) </pre>

Solution:

Reagent: NaOH = strong base + nucleophile ( $OH^{-}$ is attacking)
Mechanism: Nucleophilic acyl substitution (ester hydrolysis)
- $OH^{-}$ attacks the ester C=O
- Tetrahedral intermediate forms
- Alkoxide $(R - C H - O^{}$ leaves via C-O bond cleavage

Why saponification is irreversible:

Carboxylate $(RCOO^{-})$ is stabilized by delocalization
Reverse reaction would require $O^{2}$ form attack at C=O (bad nucleophile)
Very thermodynamically favorable

MCAT Strategy: Saponification = ester hydrolysis in basic conditions. Always produces carboxylate salt + alcohol. Often appears with triglycerides or phospholipids in biochemistry passages.

</details> <details> <summary>Example 4: Why amides are resistant to hydrolysis (the amide resonance effect)</summary>

Question: Explain why peptide bonds (amides) require enzymatic hydrolysis in the body, while esters can be hydrolyzed chemically. Use resonance to justify your answer.

<pre> Peptide bond (amide): R-C(=O)-NH-R' Ester: R-C(=O)-O-R' </pre>

Solution:

Amide resonance:
- Nitrogen lone pair ON THE SAME ATOM as the C=O
- N can donate electron density into π system of C=O
- Two resonance forms:
  - Form 1: C=O with negative charge on O
  - Form 2: C-O with positive charge on N (C= $N^{+}$ )
- Result: Partial double-bond character in the C-N bond (~40%)
Consequence of amide resonance:
- C=O electrophilicity decreases (less polarized)
- C-N bond becomes stronger (shorter, more rigid)
- Nucleophile cannot easily attack C=O
- Even if attack occurs, $NH_{2}^{-}$ is an extremely poor leaving group

MCAT Strategy: "Peptide bonds are resistant to hydrolysis" = amide resonance makes them resistant. This is a key concept linking organic chemistry to biochemistry. If a question asks "Why don't enzymes need help to break ester/phosphoester bonds?" → Esters are already labile, don't need enzyme stabilization.

</details>

Benzene: 6 $\pi$ electrons ( $n = 1$ ) ✓
Cyclopentadienyl anion: 6 $\pi$ electrons ✓
Cyclooctatetraene: 8 $\pi$ electrons → anti-aromatic (if planar)

Electrophilic Aromatic Substitution (EAS)

Substituent type	Effect on ring	Directs to
$-OH$ , $-NH_2$ , $-OR$	Activating	ortho/para
$-CH_3$ , alkyl	Activating (weak)	ortho/para
$-NO_2$ , $-CF_3$	Deactivating	meta
Halogens ( $-Cl$ , $-Br$ )	Deactivating BUT	ortho/para

Lab Techniques on the MCAT

Distillation: Separates by boiling point
Extraction: Separates by solubility (aqueous vs. organic layer)
Chromatography: Separates by polarity (TLC, column)
Recrystallization: Purifies by differential solubility at different temps

Activating groups stabilize the sigma complex and speed substitution.
Deactivating groups destabilize it and slow substitution.
Halogens are the classic exception: deactivating by induction but ortho/para directing by resonance.

Aromatics & Lab 🎯

Key Takeaways — Part 5

Aromaticity: planar + conjugated + $4n+2$ $\pi$ electrons
Activators → ortho/para; Deactivators → meta (except halogens: deactivating but ortho/para)
Know lab separation techniques — the MCAT loves "which technique would you use to..." questions
Treat substitution patterns as resonance/inductive effects, not memorization alone.

Worked Examples — Aromatic Chemistry & Lab Techniques

<details> <summary>Example 1: Predict EAS products with a halogen substituent</summary>

Question: Chlorobenzene is nitrated with HNO $_3$ /H $_2$ SO $_4$ . Which major products form?

Solution:

Chlorine is deactivating by induction but ortho/para directing by resonance.
Nitration therefore occurs mainly at ortho and para positions.
Sterics favor para over ortho.

Major products: o-nitrochlorobenzene and p-nitrochlorobenzene (para usually higher).

MCAT tip: Halogens are the classic exception: deactivating yet ortho/para directing.

</details> <details> <summary>Example 2: Choose a practical separation method</summary>

Question: A mixture contains benzoic acid, anisole, and toluene. Best method to isolate benzoic acid?

Solution:

Benzoic acid is acidic and can be converted to water-soluble benzoate.
Add NaHCO $_3$ or NaOH to extract benzoic acid into the aqueous layer.
Separate layers, then acidify aqueous phase to precipitate benzoic acid.

Best method: acid-base extraction.

MCAT tip: If one component is acidic or basic, extraction is often superior to distillation/TLC for bulk separation.

</details> <details> <summary>Example 3: Determine aromaticity correctly</summary>

Question: Is cycloheptatriene aromatic?

Solution:

Aromaticity needs planarity, full conjugation, and $4n+2$ pi electrons.
Neutral cycloheptatriene contains an sp $^3$ carbon, so conjugation is interrupted.
Therefore it is nonaromatic.

Related high-yield contrast: the tropylium cation (C $_7$ H $_7^+$ ) is aromatic with 6 pi electrons.

MCAT tip: Huckel count alone is not enough; check continuous p-orbital overlap.

</details>

Chemical shift ( $\delta$ ): TMS = 0 ppm (reference)
Alkyl: 0.5-2.0 ppm
Next to C=O: 2.0-2.5 ppm
Next to O or N: 3.0-4.0 ppm
Aromatic: 6.5-8.0 ppm
Aldehyde H: 9.0-10.0 ppm
Carboxylic acid H: 10-12 ppm

Splitting (n+1 rule)

A proton with $n$ equivalent neighboring protons splits into $n + 1$ peaks.

Triplet: 2 neighbors
Quartet: 3 neighbors

Integration and Signal Counting

Integration gives relative proton counts for each signal.
Number of unique proton environments gives number of distinct $^1$ H NMR signals.
Symmetry can reduce the number of observed signals.

Spectroscopy 🎯

Key Takeaways — Part 6

IR: Broad O-H (3200-3600 for alcohol, 2500-3300 for acid) and sharp C=O (~1715)
NMR: Chemical shift tells you environment, splitting tells you neighbors
n+1 rule: number of peaks = neighbors + 1
Mass spec: molecular ion peak (M $^+$ ) gives molecular weight
Use all clues together: IR functional groups + NMR environment + mass constraints.

Worked Examples — Spectroscopy (NMR, IR, Mass Spec)

<details> <summary>Example 1: Identify a carboxylic acid from IR</summary>

Question: A spectrum shows a strong peak near 1715 cm $^{-1}$ and a very broad band from 2500 to 3300 cm $^{-1}$ . Which functional group is most likely present?

Solution:

A strong 1715 cm $^{-1}$ signal suggests C=O.
A very broad 2500 to 3300 cm $^{-1}$ signal is characteristic of acidic O-H.
Together, these strongly indicate a carboxylic acid.

MCAT tip: Carbonyl plus very broad low O-H region is the classic COOH fingerprint.

</details> <details> <summary>Example 2: Solve a simple proton NMR pattern</summary>

Question: A molecule gives two signals: quartet integrating to 2H at 3.6 ppm, and triplet integrating to 3H at 1.2 ppm. What fragment is present?

Solution:

Quartet (2H) means that proton set sees 3 neighboring equivalent H.
Triplet (3H) means that proton set sees 2 neighboring equivalent H.
Combined pattern is the classic ethyl group CH $_3$ -CH $_2$ .
The 3.6 ppm shift for CH $_2$ suggests it is next to oxygen.

Likely fragment: CH $_3$ CH $_2$ O-.

MCAT tip: Triplet/quartet with 3H/2H integration is a fast ethyl identifier.

</details> <details> <summary>Example 3: Use formula and NMR to identify tert-butanol</summary>

Question: Formula C $_4$ H $_{10}$ O. NMR shows a 9H singlet near 1.2 ppm and a broad 1H signal that disappears with D $_2$ O. Identify the compound.

Solution:

9H singlet indicates three equivalent methyl groups attached to one carbon.
D $_2$ O-exchangeable 1H indicates an O-H proton.
Structure that matches is (CH $_3$ ) $_3$ C-OH.

Compound: tert-butanol (2-methyl-2-propanol).

MCAT tip: D $_2$ O disappearance confirms exchangeable protons like O-H or N-H.

</details> <details> <summary>Example 4: Interpret a key mass-spec fragment</summary>

Question: A spectrum has M $^+$ at m/z 92 and a strong fragment at m/z 91. What structural motif is likely present?

Solution:

m/z 91 is the tropylium/benzyl cation signal.
This peak strongly suggests a benzyl-containing structure.
A common case is toluene-like or alkylbenzene compounds that form the stable C $_7$ H $_7^+$ ion.

MCAT tip: m/z 91 is one of the highest-yield aromatic fragmentation clues.

</details>

Step 1 — Recall the rule. Compare the solution pH to each ionizable group's p $K_a$ :

If pH < p $K_a$ , the group is mostly protonated.
If pH > p $K_a$ , the group is mostly deprotonated.

Step 2 — Apply it to lysine. The lysine side chain is a basic amine. At pH 7.4 < p $K_a$ 10.5, it is protonated as $-\text{NH}_3^+$ , so it carries a +1 charge.

Step 3 — Apply it to glutamate. The glutamate side chain is a carboxylic acid. At pH 7.4 > p $K_a$ 4.1, it is deprotonated as $-\text{COO}^-$ , so it carries a –1 charge.

Step 4 — Connect to function. At physiological pH, basic residues (Lys, Arg) tend to be positive and acidic residues (Asp, Glu) negative; these opposite charges form salt bridges that stabilize protein structure and mediate substrate binding — a recurring biochemistry link.

MCAT takeaway: Charge follows the pH-vs-p $K_a$ comparison. Below p $K_a$ → protonated; above p $K_a$ → deprotonated. At pH 7.4, Lys/Arg are typically +1, Asp/Glu are –1, and His (p $K_a \approx 6$ ) sits near the transition, which is why it is a flexible catalytic residue.

Final Review 🎯

Organic Chemistry — Complete! ✅

Master the reaction decision chart, functional groups, and stereochemistry. These connect directly to amino acid and enzyme chemistry in Biochemistry.

The strongest MCAT performance comes from mechanism-first thinking, not memorizing isolated reactions.

Water can attack from either face (above or below the plane)

~50% attack from above → (S) enantiomer

~50% attack from below → (R) enantiomer

Result: Racemic mixture (although sometimes slightly favor one direction)

$H_{2}SO_{4}$ : Catalyst for esterification, not reduction

Zn/HCl: Wolff-Kishner reduction (specific for ketones to alkanes)

Correct: PCC (pyridinium chlorochromate) — stops at aldehyde

Alternative: DMP (Dess-Martin Periodinane), IBX, Swern oxidation — all work

2-butenal (crotonaldehyde)

Key question: Is there a leaving group on the carbonyl carbon?

CHO carbon is bonded to: C, C, O (no leaving group like Cl, $OCH_{3}$ , OAc, etc.)

Answer: Nucleophilic addition (NA)

Mechanism:

Methylamine acts as nucleophile, attacks C=O
Intermediate: C-OH tetrahedral intermediate
Lone pair on N attacks C, OH leaves
Product: Imine ( $CH_{3}-N$ = $C(CH_{3})_{2}$ or iminium salt initially)

Final product: N-methylpropan-2-imine or acetone methyl imine

pK_{a}(conjugate acid)

NH_{2}^{-}

pK_{a}(conjugate acid)

≈ 35 (ammonia) → terrible leaving group

Why does the ester react?

Aniline is a strong nucleophile (aromatic amine with lone pair on N)
Although $NH_{2}^{-}$ is a poor leaving group, the ester is reactive enough
Once the amide forms, it is resistant to further reaction (poor leaving group protects it)

Product: $CH_{3}-C$ (=O) $-NH-C_{6}H_{5}$ (N-phenylacetamide or N-acetylaniline)

Carboxylate ion

(RCOO^{-})

is formed

Stoichiometry: 1 triglyceride + 3 NaOH → 1 glycerol $(HOCH_{2}CHOH CH_{2}OH)$ + 3 sodium carboxylates (RCOONa, "soap")

Products:

Glycerol: 3-carbon backbone (reusable for biodiesel)
Sodium carboxylates: Long-chain salts (e.g., sodium stearate $C_{17}H_{35}COONa$ ) = SOAP!
pH increases (basic product)

Ester comparison:

Oxygen lone pair is FARTHER from C=O (on neighboring carbon)
Resonance is WEAK
C=O remains strongly electrophilic
Chemical hydrolysis (mild acid/base) works fine

Why peptides need enzymes:

Proteases stabilize the tetrahedral intermediate
Enzymes position water & catalytic residues
Lower activation energy enough to make hydrolysis significant at body temp

Spectroscopy

Spectroscopy - Complete Interactive Lesson

Part 1: Functional Groups & Nomenclature

Organic Chemistry for the MCAT

Must-Know Functional Groups

Stereochemistry

R/S Assignment (Cahn-Ingold-Prelog)

Stereochemical Relationships You Must Distinguish

Key Takeaways — Part 1

Worked Examples — Functional Groups & Stereochemistry

Part 2: Stereochemistry

Organic Chemistry for the MCAT

Substitution vs. Elimination Decision Tree

Key Points

Part 3: Substitution & Elimination

Organic Chemistry for the MCAT

Carbonyl Reactivity

Aldol Condensation

Part 4: Carbonyl Chemistry

Organic Chemistry for the MCAT

Reactivity Order (most reactive → least)

Why? The better the leaving group, the more reactive.

Part 5: Carboxylic Acid Derivatives

Organic Chemistry for the MCAT

Aromaticity Rules (Huckel)

Part 6: Spectroscopy & Structure

Organic Chemistry for the MCAT

IR Spectroscopy — Key Absorptions

H NMR — Quick Guide

Part 7: Review & MCAT Practice

Organic Chemistry for the MCAT

Highest-Yield MCAT Organic Topics

Amino Acid Side Chain Chemistry (bridges to Biochem)

Test-Day Organic Workflow

Worked Example — Predicting an Amino Acid's Charge at a Given pH

Fast Test-Day Decision Rules

Key Takeaways — Part 2

Worked Examples — Substitution & Elimination

Key Carbonyl Reactions

MCAT High Yield: Reducing Agents

Nucleophilic Addition vs Acyl Substitution

Key Takeaways — Part 3

Worked Examples — Carbonyl Chemistry

Key Interconversions

Biochemistry Connection

Core Mechanistic Pattern

Key Takeaways — Part 4

Worked Examples — Carboxylic Acid Derivatives

Electrophilic Aromatic Substitution (EAS)

Lab Techniques on the MCAT

EAS Logic Shortcuts

Key Takeaways — Part 5

Worked Examples — Aromatic Chemistry & Lab Techniques

Splitting (n+1 rule)

Integration and Signal Counting

Key Takeaways — Part 6

Worked Examples — Spectroscopy (NMR, IR, Mass Spec)

Organic Chemistry — Complete! ✅