3.1 Functional Groups

Functional groups are the reactive sites on organic molecules. They determine how a compound behaves in chemical reactions, what it dissolves in, and how it interacts with other molecules. Since alkanes (the focus of this unit) are defined by having no functional groups beyond C–H and C–C single bonds, understanding what functional groups look like helps you recognize what makes other compound families different.

Functional Groups in Organic Compounds

Key Functional Groups in Organics

A functional group is a specific arrangement of atoms within a molecule that determines its characteristic reactivity. Here are the major ones you need to know:

• Alkenes ($C=C$) contain a carbon-carbon double bond. The double bond includes a π bond, which makes alkenes electron-rich and reactive toward electrophiles. Examples: ethene, propene, butene.
• Alkynes ($C \equiv C$) contain a carbon-carbon triple bond (one σ bond and two π bonds). The extra electron density makes them even more reactive toward addition reactions than alkenes. Examples: ethyne (acetylene), propyne, butyne.
• Arenes are aromatic compounds built around benzene rings. The six π electrons are delocalized across the ring, which gives arenes unusual stability. Rather than undergoing addition reactions (like alkenes), arenes prefer electrophilic aromatic substitution, where a hydrogen on the ring is replaced. Examples: benzene, toluene, naphthalene.
• Carbonyl-containing groups all share a $C=O$ (carbonyl) unit, but differ in what's attached to the carbonyl carbon. That difference dramatically changes their reactivity:
  • Aldehydes ($RCHO$) have the carbonyl at the end of a carbon chain. The terminal position makes the carbonyl carbon more accessible to nucleophiles. Examples: formaldehyde, acetaldehyde, benzaldehyde.
  • Ketones ($RCOR'$) have the carbonyl between two carbon groups. They also undergo nucleophilic addition, but are slightly less reactive than aldehydes because the flanking carbon groups donate electron density and create steric hindrance. Examples: acetone, butanone, cyclohexanone.
  • Carboxylic acids ($RCOOH$) combine a carbonyl with a hydroxyl group ($OH$). The $OH$ makes them acidic because the conjugate base (carboxylate anion) is resonance-stabilized. They can form esters and amides through condensation reactions. Examples: acetic acid, benzoic acid, stearic acid.
  • Esters ($RCOOR'$) have a carbonyl bonded to an alkoxy group ($OR'$). They're commonly formed from carboxylic acids and alcohols, and they're prone to hydrolysis (breaking back apart with water). Examples: ethyl acetate, methyl benzoate.
  • Amides ($RCONH_2$, $RCONHR'$, or $RCONR'R''$) have a carbonyl bonded to nitrogen. They can be primary, secondary, or tertiary depending on how many carbon groups are on the nitrogen. Amides are the most stable of the carbonyl derivatives because nitrogen donates electron density into the carbonyl through resonance. Examples: acetamide, N-methylacetamide.

Influence of Functional Groups on Reactivity

The type of functional group dictates which reactions a molecule undergoes. Here's how the major groups compare:

Alkenes and alkynes react through addition reactions. Their π bonds act as nucleophilic sites that attract electrophiles like halogens ($Br_2$), hydrogen halides ($HBr$), and water (in acid-catalyzed hydration). Hydrogenation (adding $H_2$ with a metal catalyst) converts them to alkanes by breaking the π bond and forming new C–H bonds.

Arenes undergo electrophilic aromatic substitution rather than addition. The delocalized π system is so stable that the ring "prefers" to substitute a hydrogen atom rather than break its aromaticity. Common examples include halogenation, nitration, sulfonation, and Friedel-Crafts alkylation/acylation.

Carbonyl compounds vary in reactivity depending on what's attached to the $C=O$:

• Aldehydes and ketones undergo nucleophilic addition at the electrophilic carbonyl carbon. Nucleophiles like Grignard reagents, hydride reducing agents, and amines attack this carbon. They also participate in condensation reactions (aldol, Claisen).
• Carboxylic acids are acidic (typical $pK_a$ around 4–5) and undergo esterification with alcohols and amide formation with amines.
• Esters undergo hydrolysis (acid- or base-catalyzed) to regenerate carboxylic acids and alcohols, and can participate in transesterification.
• Amides are the least reactive carbonyl derivative under mild conditions, but they can be hydrolyzed under strongly acidic or basic conditions.

The polarity of a functional group also affects physical properties. Polar groups like $OH$ and $C=O$ increase water solubility and raise boiling points through hydrogen bonding or dipole-dipole interactions.

Classification of Organic Compound Families

Each family has a general molecular formula and a naming suffix that tells you which functional group is present:

• Alkenes: $C_nH_{2n}$ — suffix "-ene"
  • Example: $CH_2=CH_2$ is ethene, the simplest alkene
• Alkynes: $C_nH_{2n-2}$ — suffix "-yne"
  • Example: $CH \equiv CH$ is ethyne (acetylene)
• Arenes: contain one or more benzene rings — named as benzene derivatives
  • Example: $C_6H_5CH_3$ is toluene (methylbenzene)
• Aldehydes: $C_nH_{2n}O$ — suffix "-al"
  • Example: $CH_3CHO$ is ethanal (acetaldehyde)
• Ketones: $C_nH_{2n}O$ — suffix "-one"
  • Example: $CH_3COCH_3$ is propanone (acetone)
• Carboxylic acids: $C_nH_{2n}O_2$ — suffix "-oic acid"
  • Example: $CH_3COOH$ is ethanoic acid (acetic acid)
• Esters: $C_nH_{2n}O_2$ — named as alkyl + carboxylate
  • Example: $CH_3COOC_2H_5$ is ethyl ethanoate (ethyl acetate)
• Amides: — suffix "-amide"
  • Example: $CH_3CONH_2$ is ethanamide (acetamide)

Notice that aldehydes and ketones share the same general formula ($C_nH_{2n}O$), and carboxylic acids and esters also share one ($C_nH_{2n}O_2$). You can't distinguish them by molecular formula alone. You need to look at the connectivity of atoms, which is exactly why structural formulas matter.

Structural Considerations in Organic Compounds

Three concepts tie together how functional groups relate to molecular structure:

• Functional groups are the specific atom arrangements that determine a molecule's characteristic reactions. Everything else in the molecule (the carbon backbone, the C–H bonds) is relatively unreactive by comparison.
• Isomerism occurs when molecules share the same molecular formula but differ in how their atoms are connected or arranged in space. For example, ethanol ($CH_3CH_2OH$) and dimethyl ether ($CH_3OCH_3$) are both $C_2H_6O$, but they have completely different functional groups and properties. This is why identifying the functional group is more informative than just knowing the formula.
• Hybridization describes how atomic orbitals mix to form bonding orbitals, and it directly affects molecular geometry. A carbon in a $C=C$ double bond is $sp^2$ hybridized (trigonal planar, ~120° bond angles), while a carbon in a $C \equiv C$ triple bond is $sp$ hybridized (linear, 180°). Alkane carbons, by contrast, are $sp^3$ (tetrahedral, ~109.5°). Knowing the hybridization helps you predict bond angles, molecular shape, and reactivity.