Alkanes, Cycloalkanes, and Conformations — Study Notes
Organic molecules: hydrocarbons, alkanes, and basic nomenclature
Organic compound definition: a compound that contains carbon, with carbon as the central atom; if the compound also has hydrogen attached to carbon, it’s an organic compound.
Hydrocarbon: an organic compound that contains only carbon and hydrogen (no oxygen, nitrogen, etc.).
Distinction: carbohydrates (e.g., sugar) are different from hydrocarbons (e.g., hydrocarbons are generic name for carbon–hydrogen compounds without other heteroatoms).
Alkane basics (a subfamily of hydrocarbons):
Alkanes are hydrocarbons with only single bonds between carbon atoms; all carbons are sp^3 hybridized.
sp^3 hybridization meaning: carbon forms four single bonds (to four substituents, which can be hydrogen or carbon). No double or triple bonds.
Consequence: each carbon in an alkane forms only single bonds; no pi bonds in the backbone.
Simple alkanes: naming and basic structures
Methane (CH4): carbon at center with four hydrogens (CH4).
Ethane (C2H6): backbone of two carbons, each bonded to three hydrogens; all hydrogens and carbons are saturated; formula: .
Propane (C3H8): carbon backbone of three; end carbons each have three hydrogens, middle carbon has two hydrogens; formula: .
Butane (C4H10): straight-chain (acyclic) or branched forms exist; condensed forms shown, e.g., CH3-CH2-CH2-CH3 or CH3-CH(CH3)-CH_3 (isobutane).
General condensed Lewis structures: show carbon centers with attached hydrogens; Lewis structure emphasizes bonding electrons; for hydrocarbons, all electrons are involved in bonding (no lone pairs on C or H).
General formula for acyclic (non-cyclic) alkanes
For an alkane with n carbons:
This arises because each carbon needs four bonds; in a straight chain, there are two terminal carbons contributing 3 hydrogens each and internal carbons contributing 2 hydrogens; overall lead to 2n+2 hydrogens.
Example checks: n=1 (CH4) gives H4; n=2 (C2H6) gives H6; n=3 (C3H8) gives H8; n=4 (C4H{10}) gives H_{10}.
Constitutional isomers of alkanes (same formula, different connectivity)
Pentane (C5H{12}) has three constitutional isomers:
n-pentane (a straight chain),
isopentane (2-methylbutane),
neopentane (2,2-dimethylpropane).
All three share the same molecular formula but differ in carbon connectivity; they are constitutional (structural) isomers.
For C5H{12}, these three are explicitly illustrated in class notes:
n-pentane: CH3-CH2-CH2-CH2-CH_3
isopentane (2-methylbutane): CH3-CH(CH3)-CH2-CH3
neopentane (2,2-dimethylpropane): a central quaternary carbon bound to four CH_3 groups
General idea: different ways to arrange the same number of carbons and hydrogens yield constitutional isomers; these are non-identical in bonding pattern but identical in formula.
Condensed Lewis structures versus full drawings
Condensed Lewis form vs full Lewis structure: condensed form lists the atoms and their immediate attachments (e.g., CH3-CH2-CH_3), while full Lewis structures show all bonds and electron pairs.
For butane (C4H{10}) you can present as:
Linear: CH3-CH2-CH2-CH3
Branched (isobutane): CH3-CH(CH3)-CH_3
The condensed form helps visualize connectivity; full Lewis shows bonding electrons explicitly; both describe the same molecule.
Short-hand (skeletal) representations and cautions
In shorthand, carbons are at endpoints and intersections; hydrogens on these carbons are generally not drawn, unless the molecule is very small.
For example, butane often written as CH3-CH2-CH2-CH3 or as a zigzag with implicit hydrogens; shorthand helps readability for larger molecules.
General formula for monocyclic alkanes (cycloalkanes)
Monocyclic alkanes are alkanes with one ring; formula differs from acyclic by ring closure.
General formula for monocyclic alkanes:
Smallest monocycle: cyclopropane (C3H6).
Examples: cyclobutane (C4H8), cyclopentane (C5H{10}), cyclohexane (C6H{12}).
In cycloalkanes, every carbon is sp^3 hybridized and the ring closes by removing two hydrogens to form a C–C bond that completes the ring.
Cycloalkane naming and substituents
Nomenclature basics: prefix cyclo- is added to the base name of the ring (e.g., cyclopentane).
If there is one substituent on the ring, you usually do not need to specify the number 1 (e.g., methylcyclopentane).
If there are multiple substituents, or substituents on different carbons, numbers are assigned to give the lowest set of locants, and substituents are named in alphabetical order (ignoring prefixes like di-, tri-, etc., and ignoring cyclo in alphabetizing but including iso- when applicable).
For cycloalkanes with multiple substituents, the substituents can be placed on different ring carbons, and the substituent numbers are chosen to minimize the sum of locants; for example, 1-ethyl-3-methylcyclopentane vs other possible placements.
Substituent naming: alkyl groups derived from alkanes
Replace one H on an alkane with a substituent (an alkyl group): alkyl groups attach to the parent chain.
Common alkyl groups (with their formulas when attached):
Methyl: —CH3 (from methane, CH4)
Ethyl: —CH2CH3 (from ethane, C2H6)
Propyl: —CH2CH2CH3 (from propane, C3H_8)
Isopropyl: —CH(CH3)2 (propyl isomer, same formula as n-propyl but different connectivity)
Butyl: —C4H9 (from butane, C4H{10})
Secondary butyl (sec-butyl): attachment from a secondary carbon
Isobutyl: (CH3)2CHCH2–
Tert-butyl (t-butyl): attachment from a tertiary carbon
Important notation and prefixes:
The IUPAC substituent prefixes include isopropyl, isobutyl, sec-butyl, tert-butyl, etc.
When alphabetizing substituents, drop prefixes such as di- and tri-, and drop secondary/tertiary prefixes for alphabetization; iso- remains part of the name (e.g., isopropyl, isobutyl).
For multi-substituent compounds, use prefixes like di-, tri- in the name to indicate multiple identical substituents on different carbons (e.g., 2,2-dimethyl).
Examples of substituent naming on alkanes
Propane derivatives: two different propyl isomers are formed when attaching a substituent to a propane molecule depending on which hydrogen is removed; substituents include propyl and isopropyl.
Butane derivatives: possible substituents include butyl (on terminal carbon) and sec-butyl (on either terminal or internal carbon), with isobutyl and tert-butyl providing other attachment patterns.
The IUPAC approach: identify the longest carbon chain (the parent), then name substituents on that chain with lowest locants, then order substituents alphabetically (ignoring prefixes like di/tri and ignoring cyclo when discussing cyclic systems, but including iso-prefixes when applicable).
Steps to name an alkane (acyclic)
Step 1: Find the longest continuous carbon chain. This becomes the parent chain (backbone).
Step 2: If there are multiple chains with the same maximum length, choose the one with the greater number of substituents (to keep substituents light and to minimize locants).
Step 3: Number the carbons in the parent chain from the end that gives the first substituent the lowest possible locant; if there is a tie, choose the direction that gives the next substituent the lowest locant, and so on.
Step 4: List substituents in alphabetical order, ignoring prefix di-, tri-, sec-, tert-, etc. Keep iso- prefixes in the alphabetization.
Step 5: If identical substituents occur on the same carbon, assign separate locants (e.g., 2,2-dimethyl).
Step 6: If a substituent is multiple atoms long, write its name before the parent chain name (e.g., 4-ethyl-2-methylhexane).
Step 7: Use hyphens to separate locants from substituent names and from the parent chain name; use a comma to separate multiple locants (e.g., 2,3-dimethylpentane).
Example naming walkthroughs
Example 1: Butane (C4H{10})
Longest chain: 4 carbons → butane (or butane-1? no, usually just butane; if branched, e.g., methylpropane is isobutane, etc.).
If branched to give two methyl groups on the second and third carbons: 2,2-dimethylpropane (neopentane) is a constitutional isomer of C5H{12}.
Example 2: Pentane derivatives
n-pentane: no substituents, just a five-carbon chain.
2-methylbutane (isopentane): methyl substituent on carbon 2.
2,2-dimethylpropane (neopentane): two methyl groups on the central carbon of the propane backbone.
Cycloalkanes: naming and numbering rules
Cycloalkanes are carbons arranged in a ring; the base formula is CnH{2n}.
For monocyclic cycloalkanes with one substituent: name as substituent-cycloalkane (e.g., methylcyclopentane); no need to prefix 1-.
If there are multiple substituents:
Number the ring to give substituents the lowest set of locants.
Use cyclo- in the base name; list substituents in alphabetical order, ignoring cyclo- and any numerical prefixes (e.g., cyclohexane with substituents ethyl and methyl would be 1-ethyl-3-methylcyclohexane, with ethyl listed before methyl).
When there are multiple identical substituents on the ring, use di-, tri-, etc., to indicate multiplicity and specify their positions (e.g., 1,3-dimethylcyclohexane).
In nomenclature examples given: cycloheptane, cyclopropane, cyclobutane, cyclohexane, cyclopentane, etc.
Conformational analysis of alkanes: rotation about C–C single bonds
Alkanes are free to rotate around C–C single bonds due to the sigma (σ) bond overlap of sp^3–sp^3 orbitals (no pi-bond lock).
Consequences: different spatial arrangements (conformations) of the same molecule can interconvert without breaking bonds.
Conformations are different spatial arrangements around a C–C bond; rotation changes how substituents are oriented in space, but not connectivity.
Ethane conformations: eclipse vs staggered
Two extreme conformations for ethane: eclipsed (or eclipsed ethane) and staggered.
Eclipse: hydrogens on adjacent carbons align with each other (front and back hydrogens are directly in line); there is more steric repulsion.
Staggered: hydrogens on adjacent carbons are offset from each other, reducing steric repulsion and lowering energy.
The energy difference between eclipsed and staggered conformations is about , with staggered being more stable.
Dihedral angle and conformations
The dihedral angle (θ) is the angle between a substituent on the front carbon and the corresponding substituent on the back carbon as you rotate around the C–C bond.
Key angular positions in the staggered/eclipsed sequence occur at 0°, 60°, 120°, 180°, 240°, 300° (or 0°, 60°, 120°, 180°, 240°, 300° depending on the reference).
In Newman projections, the front carbon is drawn as the center, with the back carbon rotated to show relative positions of substituents around the C–C bond.
In sawhorse representations, you show a three-dimensional sense with wedges and dashes to indicate coming out of or going into the plane.
Visual representations: sawhorse and Newman projections
Sawhorse representation: shows bonds in 3D with planes and dashed/wedged bonds to indicate substituents coming out of or going behind the plane; useful for visualizing actual 3D orientation.
Newman projection: view down the C–C bond; front carbon at the center; back carbon at the circumference; substituents placed around; allows easy read of dihedral angles (0°, 60°, 120°, 180°, etc.).
Dihedral angles and conformational energies can be tracked by rotating one substituent around the C–C bond while keeping the other fixed.
Relative stability and energy profile of conformations
The eclipsed conformation is less stable (higher energy) due to steric repulsion between adjacent hydrogens (or substituents).
The staggered conformation is more stable (lower energy) because substituents are anti-periplanar and minimize repulsive interactions.
When rotating from eclipsed to staggered, energy drops by about
The energy profile for rotation about a C–C single bond typically shows repeating cycles of high-energy eclipsed peaks and low-energy staggered troughs every 60°.
Going from simple to more complex conformations (future topics)
In larger alkanes, the bulkiness of substituents (e.g., tert-butyl, isopropyl, etc.) affects the energy landscape and preferred conformations.
In later chapters, stereochemistry and more advanced conformational analyses (including substituted alkanes) will be discussed.
If given a molecular weight, you can determine the number of carbons using the general formula and the atomic weights: C = 12, H = 1; for an alkane: $$CnH{2n+2} \