Foundational Concepts › Organic Chemistry

Functional Groups - Overview

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Functional Groups - Overview

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Functional groups

Sets of atoms and bonds characterized by a predictable chemical behavior

Overview

Functional group

  • Set of atoms and bonds characterized by a predictable chemical behavior

Formal Charge (FC)

  • Used to identify lone pairs of heteroatoms in line structures
  • FC = number of valence electrons – (number of nonbonding electrons + number of bonds)

Structures of functional groups of heteroatoms

Halides

  • Alkyl halide = R-X

Oxygen

  • Alcohol = R-OH
  • Ether = R-OR'
  • Ketone = R-carbonyl-R'
  • Aldehyde = R-carbonyl-H
  • Acetal = carbon bonded to OR, OR', R'', R'''
  • Carboxylic acid = R-carboxyl
  • Anhydride = R-carbonyl-O-carbonyl-R'
  • Ester = R-carbonyl-OR'

Nitrogen

  • Amine = nitrogen bonded to R, R', R''
  • Imine = carbon double-bonded to NR

Sulfur

  • Thiol = R-SH
  • Sulfide = R-SR'

Halides and oxygen

  • Acyl halide = R-carbonyl-X

Oxygen and nitrogen

  • Amide = carbonyl bonded to an amine

Oxygen and sulfur

  • Sulfone = sulfur double-bonded to two oxygens

Full-Length Text

  • Here, we will learn about functional groups.
  • Denote that a functional group is a set of atoms and bonds that are characterized by a predictable chemical behavior, and that
    • Functional groups determine how molecules change in chemical reactions.

At this point, let's look at functional groups with heteroatoms.

  • First, remind ourselves of the heteroatom definition: they are any atom other than carbon or hydrogen.
    • To be proficient in our understanding of functional groups we must:
  1. Determine the electron count of the central atom (here, we are focusing on heteroatomic functional groups).
  2. Recognizing the structure of the functional group.

Step 1.

  • It would be easy to recognize the electron count of a heteroatom, if the line structures showed lone pairs, but this is not always the case.
  • So write that we use the formal charge of an atom to identify its number of lone pairs.
  • Write out that the formula for formal charge is:
    • (Valence electrons of an Atom) minus (the number of Nonbonding electrons + the number of Bonds).
    • This is the atomic valence electrons minus the electrons assigned to a bonded atom in a molecule.
  • Then that:

    • nonbonding electrons = # valence electrons – (# bonds + formal charge)

So now, let's determine the electron count of heteroatoms within an actual molecule: MDMA (3,4-methylenedioxy-methamphetamine). MDMA has the formula C11H15NO2.

  • Draw it as follows:
    • A sideways five-membered ring with two oxygens, and with
    • A six-membered ring connected on its right side.
  • Draw three double bonds in the ring.
  • In zigzag fashion, connect a backbone of two carbons to the ring.
  • Add a methyl substituent to the second carbon.
  • And add a nitrogen to the right.
  • Add a hydrogen above the nitrogen.
  • And add a methyl group to the right of the nitrogen.
  • Always draw the substituents pointing away from the other bonds.

Next, identify the number of electrons connected to an oxygen in the five-membered ring.

  • To do so, first, determine oxygen's valence electron number, which is 6, because oxygen is in group 6 of the periodic table.
  • Next, determine its number of Nonbonding electrons, which is:
    • Valence electrons: 6 minus Bonds: 2 + Formal charge: 0
    • We know this because no formal charge is shown on the oxygen in the molecule.
    • So, # nonbonding electrons = 4 and
    • 4 Nonbonding e- = 2 Lone pairs

Similarly, let's address nitrogen as a central atom.

  • Nitrogen has 5 valence electrons (it's in group 5).
    • Nonbonding electrons show that valence electrons: 5 minus Bonds: 3 + Formal charge: 0
    • We know this because no formal charge is shown on the nitrogen in the molecule.
    • So, # nonbonding electrons = 2 and
    • 2 Nonbonding e- = 1 Lone pair

Step 2.

  • Next, write that we need to be able to recognize the structure of the functional groups of heteroatoms.
  • Write that we'll cover functional groups for the following heteroatoms:
    • Sulfur
    • Nitrogen
    • Oxygen
    • Halides
  • Start a Venn diagram.
    • Add a circle for Oxygen.
    • Add circles for Halides, Nitrogen, and Sulfur that overlap with Oxygen.
  • Under Halides, write:
    • Alkyl halide and represent the group as R–X.
    • We use an R to represent an attachment containing carbon or hydrogen, and the designations of ' (prime), '' (double prime), and so on to indicate that the attachments do not have to be the same.
  • Under Oxygen, write:
    • Alcohol, as an -OH (hydroxyl) group.
    • Ether, as an –OR group.
    • Ketone, as a carbon double-bonded to an oxygen (carbonyl group) and bonded to two R groups.
    • Aldehyde, as a carbonyl bonded to a hydrogen and an R group.
    • Acetal, as two -OR groups bonded to the same carbon.
    • Carboxylic acid, as a carbonyl group bonded to a hydroxyl group (a carboxyl group) and an R group.
    • Anhydride, as two carbons, each double-bonded to an oxygen, that are connected with a single bond to the same oxygen.
    • Ester, as a carbonyl bonded to an –OR group.
  • Under Nitrogen, write:
    • Amine, as an N bonded to three R groups.
    • Imine, as carbon double-bonded to an -NR group.
  • Under Sulfur, write:
    • Thiol, as an –SH group.
    • Sulfide, as an –SR group.
  • In the overlap between Halides and Oxygen, write:
    • Acyl halide, as a carbonyl bonded to an X.
  • In the overlap between Oxygen and Nitrogen, write:
    • Amide, as a carbonyl bonded to an amine group.
  • In the overlap between Oxygen and Sulfur, write:
    • Sulfone, as a sulfur double-bonded to two oxygens.

Now, let's go back to MDMA and use what we've learned to identify the functional groups that involve oxygen and nitrogen.

  • For oxygen, indicate an acetal for the functional group that involves -O-C-O-.
  • For nitrogen, indicate an amine for its functional group.