Alkanes & Cycloalkanes

Alkanes & Cycloalkanes — IUPAC Nomenclature

Part 1 of 7 — Nomenclature

Alkanes are saturated hydrocarbons: every carbon is $sp^3$-hybridized and bonded to the maximum number of hydrogens, so the general formula for an acyclic (open-chain) alkane is $C_nH_{2n+2}$. A single ring removes two hydrogens, giving cycloalkanes the formula $C_nH_{2n}$.

A reliable, unambiguous name is not optional in organic chemistry — "hexane" and "2,2-dimethylbutane" share the formula $C_6H_{14}$ but are entirely different compounds (constitutional isomers). The IUPAC system encodes the exact connectivity of a molecule into its name, so that any chemist can redraw the structure from the name alone.

The unbranched (straight-chain) parents you must know cold:

The suffix -ane signals an alkane. The same numerical prefixes (meth-, eth-, prop-, …) reappear throughout organic chemistry attached to other suffixes (-ene, -yne, -ol), so memorizing them now pays off for the entire year.

The Five-Step IUPAC Algorithm

Naming a branched alkane is a deterministic procedure. Follow the steps in order and the name falls out:

1. Find the parent chain. Identify the longest continuous chain of carbons. This may not be the chain drawn horizontally — you must trace through bends. If two chains are tied in length, choose the one with more substituents.

2. Number the chain. Number the carbons consecutively, starting from the end that gives the lowest set of locants to the substituents (compare the first point of difference).

3. Identify and name the substituents. A substituent formed by removing one H from an alkane is an alkyl group, named by changing -ane to -yl: methyl ($-CH_3$), ethyl ($-CH_2CH_3$), propyl, etc.

Worked Example — Naming a Branched Alkane

Name the compound whose skeleton is a 6-carbon chain bearing a methyl group on the third carbon and ethyl groups on the third and fourth carbons (drawn from the left):

$CH_3-CH_2-C(CH_3)(C_2H_5)-CH(C_2H_5)-CH_2-CH_3$

Checkpoint — Parent Chain & Locants

Cycloalkanes

A cycloalkane is named by adding the prefix cyclo- to the parent name of the ring: a six-membered ring is cyclohexane, a three-membered ring is cyclopropane, and so on. The ring itself is usually taken as the parent.

Numbering a substituted ring: assign C1 to a substituted carbon, then number around the ring in the direction that gives the lowest set of locants to the remaining substituents. When substituents differ, the one first in alphabetical order receives the lower locant at the first point of difference.

• One substituent: no locant is needed (e.g. methylcyclohexane — the ring carbon bearing the methyl is "C1" by default).
• Two or more substituents: number to give the lowest locants, breaking ties alphabetically.

Ring vs. chain as parent: if the ring has more carbons than the largest attached chain, the ring is the parent (e.g. 1-ethyl-2-methylcyclohexane). If a chain attached to the ring is longer than the ring, the ring becomes a cycloalkyl substituent on that chain (e.g. a propyl chain bearing a cyclohexane ring is named cyclohexyl-substituted propane $\rightarrow$ "1-cyclohexylpropane" style naming, i.e. the ring is the substituent).

Worked check: A cyclopentane ring with a methyl on one carbon and a chlorine two carbons away. Start numbering at a substituted carbon and go the direction giving lowest locants. With substituents alphabetized (chloro before methyl), we get 1-chloro-3-methylcyclopentane rather than "3-chloro-1-methyl…", because the lower locant set is assigned to the first-cited (alphabetically lower) substituent at the first point of difference.

Checkpoint — Constitutional Isomers & Cyclo Naming

Exit Ticket — Putting a Name Together

Conformational Analysis

Part 2 of 7 — Conformations of Acyclic Alkanes

A single $C-C$ $\sigma$ bond allows nearly free rotation. The different three-dimensional arrangements a molecule adopts by rotating about such bonds are called conformations (or conformers / rotamers). Crucially, interconverting conformers does not break any bonds — they are not isomers in the structural sense and cannot be separated under normal conditions.

The key variable is the dihedral (torsion) angle: the angle between two bonds on adjacent carbons when viewed down the $C-C$ axis. As this angle changes, the molecule's potential energy rises and falls. The energy differences are small (a few ), so at room temperature molecules spin rapidly through all conformations — but they spend most of their time in the lowest-energy ones.

Cyclohexane Chair Conformations

Part 3 of 7 — The Chair, Ring Flips, and Diaxial Strain

Although a flat hexagon suggests $120^\circ$ internal angles, a real cyclohexane ring is not planar. By puckering, the ring lets every carbon adopt nearly perfect tetrahedral ($109.5^\circ$) angles and keeps every adjacent $C-H$ pair staggered. The result is the famous — essentially strain-free (no angle strain, no torsional strain).

Ring Strain in Cycloalkanes

Part 4 of 7 — Angle Strain, Torsional Strain, and Ring Size

The internal angles of a flat regular polygon are fixed by geometry: $60^\circ$ for a triangle (cyclopropane), $90^\circ$ for a square (cyclobutane), $108^\circ$ for a pentagon (cyclopentane), for a hexagon. But carbon "wants" the tetrahedral angle of . The deviation from this ideal produces .

Physical Properties of Alkanes

Part 5 of 7 — Boiling Points, Branching, Solubility, and IMFs

Alkanes are nonpolar. The $C-C$ bond is between identical atoms, and the $C-H$ bond has only a tiny electronegativity difference ($\approx 0.4$), so alkanes have essentially no permanent dipole. With no dipole–dipole forces and no hydrogen bonding, the only intermolecular force available to alkanes is the weakest one: (LDF), also called van der Waals or induced-dipole/induced-dipole forces.

Reactions Workshop: Combustion & Radical Halogenation

Part 6 of 7 — Putting Reactivity to Work

Alkanes are famously unreactive ("paraffins" — parum affinis, "little affinity"): no $\pi$ bonds, no lone pairs, no polar bonds for nucleophiles or electrophiles to attack. Only two reactions matter at this level, and both involve breaking strong C–H bonds homolytically:

1. Combustion — complete oxidation with $O_2$ to $CO_2$ and , releasing large amounts of heat. The basis of alkanes as fuels.

Synthesis & Cumulative Review

Part 7 of 7 — Tying It All Together

You now hold the full toolkit for alkanes and cycloalkanes. This capstone integrates the four pillars of the unit:

1. Nomenclature — encoding exact connectivity into an unambiguous IUPAC name (longest chain, lowest locants, alphabetized substituents). The grammar that lets chemists communicate structure.
2. Conformational analysis — predicting the populated 3-D shapes from torsional and steric strain (Newman projections; anti/gauche; chair, axial/equatorial, ring flip, A-values).
3. Ring strain — explaining stability and reactivity of cycloalkanes from angle and torsional strain, quantified by heats of combustion.
4. Reactions — combustion (the fuel reaction) and free-radical halogenation (initiation/propagation/termination, with selectivity).

The connecting thread across all four is stability driven by minimizing strain and maximizing favorable orbital/steric arrangements — the same logic whether you are choosing a chair, ranking a conformer, or predicting a halogenation product.

How the Pieces Connect

A single worked thread shows the concepts reinforcing one another.

Consider trans-1,2-dimethylcyclohexane undergoing monobromination at a tertiary-like position:

• Nomenclature tells us the substrate: a cyclohexane ring (parent, 6 C beats any chain) with two methyls, trans, at C1 and C2.
• Conformational analysis tells us its shape: the trans-1,2 isomer can place both methyls equatorial (e,e) — the favored chair — minimizing 1,3-diaxial strain.
• Ring strain reminds us the six-membered ring itself is strain-free, so the ring will not open; only substitution is possible.
• Reactions let us predict that $B$ — the halogen — will abstract the most stable (most substituted) hydrogen, e.g. a ring C–H, giving predominantly one product.