Study Guide: Structure, Nomenclature, and Properties of Alkanes, Alkenes, Alkynes, Aromatics, and Alcohols

Carbon Bonding and Hydrocarbons

Carbon Bonding in Alkanes

Carbon atoms in alkanes form only single (sigma) bonds. This is due to carbon's ability to hybridize its orbitals, specifically forming four sp3 hybrid orbitals, allowing for four sigma bonds.

• sp3 hybridization: Carbon mixes one s and three p orbitals to form four equivalent sp3 orbitals.

• Sigma (σ) bonds: Single covalent bonds formed by the head-on overlap of orbitals.

• Bond angles: In alkanes, the tetrahedral geometry leads to bond angles of approximately 109.5°.

Hydrocarbon Classification

Hydrocarbons are organic compounds composed only of carbon and hydrogen. They are classified based on the types of bonds between carbon atoms:

• Alkanes: Only single bonds (saturated hydrocarbons).

• Alkenes: At least one double bond (unsaturated hydrocarbons).

• Alkynes: At least one triple bond (unsaturated hydrocarbons).

• Aromatic compounds: Contain benzene-like rings with delocalized electrons.

Nomenclature of Alkanes

Prefixes for Carbon Chain Length

The names of alkanes are based on the number of carbon atoms in the longest continuous chain. The following table summarizes the prefixes used:

Carbon atoms	Prefix
1	meth
2	eth
3	prop
4	but
5	pent
6	hex
7	hept
8	oct
9	non
10	dec

Straight-Chain Alkane Names

The following table lists the names and structures of straight-chain alkanes:

Number of Carbons	Structure	Name
1	CH4	Methane
2	CH3CH3	Ethane
3	CH3CH2CH3	Propane
4	CH3(CH2)2CH3	Butane
5	CH3(CH2)3CH3	Pentane
6	CH3(CH2)4CH3	Hexane
7	CH3(CH2)5CH3	Heptane
8	CH3(CH2)6CH3	Octane
9	CH3(CH2)7CH3	Nonane
10	CH3(CH2)8CH3	Decane

Isomerism in Alkanes

Isomers are compounds with the same molecular formula but different structures or arrangements of atoms.

• Straight-chain alkanes: All carbon atoms are in a row.

• Branched-chain alkanes: At least one carbon is attached to three or four other carbons.

• Constitutional isomers: Compounds with the same molecular formula but different connectivity of atoms.

Example: Butane (C4H10) has two isomers: n-butane (straight chain) and isobutane (branched).

Naming Alkanes (IUPAC System)

The IUPAC system provides rules for naming alkanes:

1. Find the longest continuous carbon chain (parent chain).

2. Number the carbon atoms in the main chain so that substituents have the lowest possible numbers.

3. Identify and name the substituents (alkyl groups), and assign their position numbers.

4. Combine the names and numbers, listing substituents alphabetically.

Common Alkyl Groups:

Group	Structure	Abbreviation
Methyl	CH3-	Me
Ethyl	CH3CH2-	Et
Propyl	CH3CH2CH2-	Pr
Isopropyl	(CH3)2CH-	i-Pr
Butyl	CH3CH2CH2CH2-	Bu
sec-Butyl	CH3CH2CH(CH3)-	sec-Bu
Isobutyl	(CH3)2CHCH2-	i-Bu
tert-Butyl	(CH3)3C-	t-Bu

Additional info: The position of each substituent is indicated by the number of the carbon to which it is attached.

Physical Properties and Bonding

Types of Bonds and Intermolecular Forces

• Van der Waals (London dispersion) forces: Weak attractions due to temporary dipoles in molecules.

• Polar-polar interactions: Occur between molecules with permanent dipoles.

• Hydrogen bonding: Strong dipole-dipole interaction involving hydrogen bonded to N, O, or F.

The boiling point of alcohols is much higher than that of the parent alkane due to hydrogen bonding.

Reactions of Alkanes, Alkenes, and Alkynes

Reactions of Alkanes

• Combustion: Reaction with oxygen to produce CO2 and H2O.

• Halogenation: Replacement of a hydrogen atom by a halogen (e.g., Cl2, Br2).

Reactions of Alkenes and Alkynes

• Alkenes: Undergo addition reactions at the double bond.

• Alkynes: Undergo addition reactions at the triple bond.

• Markovnikov's Rule: In the addition of HX to an alkene, the hydrogen attaches to the carbon with more hydrogens already attached.

Aromatic Compounds

Structure and Properties

• Aromaticity: Describes substances with delocalized electrons in a ring (e.g., benzene).

• Stability: Aromatic compounds are unusually stable due to resonance.

Naming Aromatic Compounds

• Substituents are named using locational descriptors: o- (ortho), m- (meta), p- (para).

• Common names: toluene (methylbenzene), aniline (aminobenzene), phenol (hydroxybenzene).

Example: The benzene ring itself may be considered a substituent group called a phenyl group.

Common Aromatic Reactions

• Nitration: Substitution of a nitro group (-NO2).

• Sulfonation: Substitution of a sulfonic acid group (-SO3H).

Types of Organic Reactions

• Addition: Atoms are added to a double or triple bond.

• Elimination: Atoms are removed, forming double or triple bonds.

• Substitution: One atom or group is replaced by another.

• Rearrangement: The structure of the molecule is rearranged.

Free Radical Halogenation: Replacement of an alkane hydrogen by a halogen, initiated by heat or light. The mechanism involves initiation, propagation, and termination steps.

Chirality and Stereochemistry

Chirality

• Chiral center: A carbon atom bonded to four different groups.

• Enantiomers: Non-superimposable mirror images.

• Optical activity: The ability to rotate plane-polarized light.

• Racemic mixture: Contains equal amounts of both enantiomers; optically inactive.

Polarimeter: Instrument used to measure optical rotation.

Alcohols

Structure and Classification

• Alcohol: Organic compound with an -OH (hydroxyl) group attached to a saturated carbon atom.

• Classification: Alcohols are classified as primary, secondary, or tertiary based on the number of carbon atoms attached to the carbon bearing the -OH group.

Physical Properties of Alcohols

• Alcohols have higher boiling points than alkanes due to hydrogen bonding.

• Alcohols with short carbon chains (methanol, ethanol) are miscible with water.

• Alcohols with four or more carbon atoms have limited solubility in water.

• Alcohols contain both hydrophilic (OH group) and hydrophobic (alkyl chain) parts.

Naming Alcohols

• Identify the longest carbon chain containing the -OH group.

• Number the chain so that the -OH group has the lowest possible number.

• Use the suffix "-ol" (e.g., ethanol, 2-propanol).

• Do not use -ol with cycloalkanes; use only for ring structures.

Example:

• Methanol: CH3OH

• Ethanol: CH3CH2OH

• 2-Propanol: (CH3)2CHOH

Additional info: Alcohols are important solvents and intermediates in organic synthesis.