20.1 Hydrocarbons

Introduction to Hydrocarbons

Hydrocarbons are the simplest organic compounds, built from just carbon and hydrogen. They serve as the foundation for all other organic molecules and show up constantly in everyday life: natural gas heating your home, the gasoline in cars, and the raw materials used to make plastics and pharmaceuticals. Understanding how they're named, how they react, and how their structures vary is the starting point for organic chemistry.

Significance of Hydrocarbons

Because carbon can form four bonds, it creates a huge variety of structures: straight chains, branched chains, and rings. These structures can include single, double, or triple bonds between carbon atoms, and each arrangement gives the molecule different physical and chemical properties.

• Hydrocarbons are the main components of fossil fuels (natural gas, petroleum, coal)
• They're the starting materials for producing plastics, synthetic fibers, pharmaceuticals, and many other industrial products
• Their structural diversity is what makes organic chemistry so broad: even small molecules with the same number of atoms can behave very differently depending on how those atoms are connected

IUPAC Nomenclature for Hydrocarbons

Naming hydrocarbons follows a system created by IUPAC (International Union of Pure and Applied Chemistry). The name tells you two things: how many carbons are in the longest chain, and what type of bonds are present.

Alkanes (saturated hydrocarbons) contain only single bonds between carbon atoms. Their general formula is $C_nH_{2n+2}$, where $n$ is the number of carbon atoms.

• The name uses a prefix for the carbon count followed by the suffix "-ane"
• Examples: methane ($CH_4$, 1 carbon), ethane ($C_2H_6$, 2 carbons), propane ($C_3H_8$, 3 carbons), butane ($C_4H_{10}$, 4 carbons)
• Common prefixes to memorize: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10)

Alkenes (unsaturated, double bonds) contain at least one carbon-carbon double bond. Their general formula for one double bond is $C_nH_{2n}$.

• The suffix changes to "-ene": ethene ($C_2H_4$), propene ($C_3H_6$)
• If there are multiple double bonds, the suffix becomes "-diene," "-triene," etc.
• For chains with 4+ carbons, a number indicates the position of the double bond (e.g., 1-butene vs. 2-butene)

Alkynes (unsaturated, triple bonds) contain at least one carbon-carbon triple bond. Their general formula for one triple bond is $C_nH_{2n-2}$.

• The suffix changes to "-yne": ethyne ($C_2H_2$), propyne ($C_3H_4$)
• Multiple triple bonds use "-diyne," "-triyne," etc.
• Position numbering works the same way as with alkenes

Reactions and Isomerism

Reactions of Alkanes vs. Alkenes

The type of bond between carbon atoms directly controls how reactive a hydrocarbon is. Single bonds are strong and stable, so alkanes don't react easily. Double and triple bonds have extra electron density that other molecules can attack, making alkenes and alkynes much more reactive.

Alkanes are relatively unreactive. Their two main reaction types are:

• Combustion: Alkanes burn in oxygen to produce carbon dioxide and water. This is why they work so well as fuels. For example, methane combustion: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$
• Substitution: In the presence of light or heat, a halogen atom (like Cl) can replace one of the hydrogen atoms on the alkane

Alkenes are more reactive because of the $C=C$ double bond. They primarily undergo addition reactions, where new atoms add across the double bond and break it into a single bond:

1. Hydrogenation: addition of $H_2$ (converts an alkene into an alkane)
2. Halogenation: addition of $X_2$ (e.g., $Br_2$), which is actually used as a test for unsaturation since the bromine color disappears
3. Hydration: addition of $H_2O$ (produces an alcohol)

Alkenes can also undergo polymerization, where many small alkene molecules link together to form long-chain polymers. Polyethylene, one of the most common plastics, is made this way from ethene.

Alkynes undergo similar addition reactions as alkenes, but because a triple bond can break in two stages, these reactions can happen in two steps. For example, hydrogenation of an alkyne can first produce an alkene, and then further hydrogenation produces an alkane.

Isomers in Hydrocarbon Molecules

Isomers are compounds that share the same molecular formula but have different arrangements of atoms. This is a big deal in organic chemistry because different arrangements mean different properties, even with the exact same atoms.

Structural isomers differ in which atoms are bonded to which. For example, $C_4H_{10}$ (butane) has two structural isomers:

• n-butane: a straight chain of four carbons
• isobutane (2-methylpropane): a branched chain with three carbons in the main chain and a methyl group on the middle carbon

These two molecules have different boiling points and slightly different physical properties despite having the same formula.

Geometric isomers (cis-trans isomers) have the same connectivity but differ in the spatial arrangement of groups around a double bond. Double bonds prevent rotation, so groups can be locked on the same side or opposite sides.

• Cis: substituents on the same side of the double bond
• Trans: substituents on opposite sides of the double bond
• Example: 2-butene ($C_4H_8$) exists as both cis-2-butene and trans-2-butene, and these have measurably different boiling points

Structural Features and Properties

Hybridization and Bonding

The type of bond a carbon forms depends on its hybridization, which is how its atomic orbitals mix before bonding. You don't need to go deep into orbital theory for intro chem, but knowing the three types and their shapes is useful:

• $sp^3$ hybridization (alkanes): carbon forms 4 single bonds, creating a tetrahedral shape with bond angles of about 109.5°
• $sp^2$ hybridization (alkenes): carbon forms a double bond plus 2 single bonds, creating a trigonal planar shape with bond angles of about 120°
• $sp$ hybridization (alkynes): carbon forms a triple bond plus 1 single bond, creating a linear shape with bond angles of 180°

The pattern is straightforward: more bonds packed between two carbons means a simpler geometry around each carbon.

Functional Groups and Reactivity

A functional group is a specific arrangement of atoms that gives a molecule its characteristic chemical behavior. In hydrocarbons, the carbon-carbon double bond ($C=C$) and triple bond ($C \equiv C$) act as functional groups. They're the reactive sites on the molecule, which is why alkenes and alkynes undergo addition reactions while alkanes (with no functional group beyond C-C and C-H single bonds) are comparatively inert.

This concept becomes even more important as you move beyond hydrocarbons. Alcohols, acids, amines, and other organic molecules are all defined by their functional groups.

Aromaticity and Stability

Aromatic compounds are a special category of cyclic hydrocarbons. Benzene ($C_6H_6$) is the classic example: a six-carbon ring where electrons are delocalized (spread evenly) across the entire ring rather than locked in alternating double bonds. This delocalization gives aromatic compounds unusual stability, making them much less reactive than you'd expect for a molecule with that many double bonds.

Aromatic rings follow Hückel's rule: a planar, cyclic molecule is aromatic if it has $4n + 2$ pi electrons (where $n$ is a non-negative integer). Benzene has 6 pi electrons, which fits $n = 1$.

Saturation and Physical Properties

Whether a hydrocarbon is saturated (all single bonds) or unsaturated (contains double or triple bonds) affects its physical properties:

• Boiling point generally increases with molecular size (more carbons = higher boiling point) because larger molecules have stronger London dispersion forces
• Branching lowers the boiling point compared to a straight-chain isomer, because branched molecules are more compact and have less surface area for intermolecular contact
• All hydrocarbons are nonpolar, so they don't dissolve in water but do dissolve in other nonpolar solvents
• At room temperature, small hydrocarbons (1-4 carbons) tend to be gases, medium ones (5-17 carbons) are liquids, and large ones (18+ carbons) are solids