Functional group

In organic chemistry, functional groups are specific groups of atoms or bonds within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of. However, its relative reactivity can be modified by nearby functional groups.

The word moiety is often used synonymously with "functional group" but, according to the IUPAC definition, a moiety is a part of a molecule that may include either whole functional groups or parts of functional groups as substructures

Quotes

• The structural features that make it possible to classify compounds into families are called functional groups. ... The chemistry of every organic molecule, regardless of size and complexity, is determined by the functional groups it contains.
  • John McMurry, Organic Chemistry 8th ed. (2012), Ch. 3 : Organic Compounds: Alkanes and Their Stereochemistry

• The question of how one chooses appropriate carbon-carbon bond disconnections is related to functional group manipulations since the distribution of formal charges in the carbon skeleton is determined by the functional group(s) present. The presence of a heteroatom in a molecule imparts a pattern of electrophilicity and nucleophilicity to the atoms of the molecule. The concept of alternating polarities or-latent polarities (imaginary charges) often enables one to identify the best positions to make a disconnection within a complex molecule.
  Functional groups may be classified as follows:
  E class: Groups conferring electrophilic character to the attached carbon (+) …
  G class: Groups conferring nucleophilic character to the attached carbon (-) …
  A class: Functional groups that exhibit ambivalent character (+ or -) …
  The positive charge (+) is placed at the carbon attached to an E class functional group … and the TM is then analyzed for consonant and dissonant patterns by assigning alternating polarities to the remaining carbons. In a consonant pattern, carbon atoms with the same class of functional groups have matching polarities, whereas in a dissonant pattern, their polarities are unlike. If a consonant pattern is present in a molecule, a simple synthesis may often be achieved.
  • George S. Zweifel and Michael Nantz, Modern Organic Synthesis (2006), Ch. 1. Synthetic Design