Functional Groups & IR Spectroscopy Notes

Functional Groups & IR Spectroscopy

Functional Groups

• Structural features within molecules that have characteristic properties and chemical behaviors.
• A functional group has a similar chemical behavior in every molecule.
• The chemistry of an organic molecule is determined by the functional groups it contains.

Hydrocarbons

• Molecules containing only carbon and hydrogen atoms.
• Contain a mixture of carbon-carbon and carbon-hydrogen single bonds (C–C and C–H) and carbon-carbon multiple bonds (C=C or C≡C).
• Alkanes do not have functional groups; C–C and C–H bonds are fairly unreactive.
• Examples include alkanes, alkenes, alkynes, and arenes/aromatic rings.

Alkanes

• Molecules consisting of only carbon and hydrogen connected by single ($\sigma$) bonds.
• General formula: $C<em>nH</em>{2n+2}$, where n is an integer.
• Also known as aliphatic compounds or saturated hydrocarbons.
• Unsaturated hydrocarbons have double or triple C–C bonds.

Classification of Alkanes

• Simplest alkanes:
  • Methane ($CH_4$)
  • Ethane ($CH<em>3CH</em>3$)
  • Propane ($CH<em>3CH</em>2CH_3$)
• Only one way to link two or three carbons in a linear fashion.
• For alkanes with more than four carbons, there are more possibilities:
  • Linear alkanes are called straight-chain or normal (n-butane).
  • Branched-chain alkanes do not have a linear connection between all carbons.
    • Example: Butane ($C<em>4H</em>8$)
    • n-butane and isobutane are constitutional isomers.

Straight-Chain Alkanes – Memorize!

Common Alkyl, Allyl, Benzyl, and Phenyl Groups

• Removal of a hydrogen from an alkane gives an ALKYL GROUP.
• Named by taking the parent name and replacing the -ane ending with -yl.
• Alkyl groups can be represented by “R”.
• R groups are used to designate any alkyl group in a molecule.
• Strategy can be used for other common hydrocarbon fragments.
• Examples:
  • Methane to methyl.
  • Ethane to ethyl (