Chemistry 121 Winter 2001 Course Notes - Principles of Chemistry II

ORGANIC NOMENCLATURE

• Confusion in organic chemistry arises due to the variety of names applied to compounds (common, trade, systematic).
• A systematic IUPAC naming system is used to uniquely name organic compounds based on their structure.
• Compounds are classified and named by:
  • The number and types of atoms present.
  • The bond types in the molecule.
  • The geometry of the molecule.

Formulae

• Molecular formula gives no explicit structural information.

• Example: $C<em>2H</em>6O$ can be ethanol or dimethyl ether.

• Structural formulae differentiate compounds by showing atom arrangement.

• Atoms after a carbon in a formula indicate elements or groups attached to that carbon.

• All carbon atoms in organic compounds must form four bonds.

  • Examples:
    • $CH<em>3CHClCCl</em>3$
    • $CH<em>3CH(CH</em>3)CH<em>2CH=CH</em>2$
    • $C<em>6H</em>5CH(NH<em>2)COOH$ (where $C</em>6H_5$ is a benzene ring and $COOH$ is an acid group)

• Bond angles are often drawn as right angles in 2D representations, but actual bond angles are rarely 90 degrees.

• Configurational structures are used when absolute geometry is important.

• Example of propane ($C<em>3H</em>8$) showing bond angles of 109°.

Nomenclature

• Compounds are classified and named based on their structure.
• The simplest classification is hydrocarbons (compounds of carbon and hydrogen).
• Hydrocarbons are classified as aliphatic or aromatic.
• Aliphatics include alkanes, alkenes, and alkynes.
• Aromatic hydrocarbons contain one or more benzene rings.

Alkanes

• Contain only C and H.
• Are saturated (contain only single bonds).
• Straight-chain alkanes are named according to the number of carbon atoms.
• Normal alkanes form a homologous series with the formula $C<em>nH</em>{2n+2}$, where n is an integer.
• Names of the first 10 alkanes:
  • 1: methane
  • 2: ethane
  • 3: propane
  • 4: butane
  • 5: pentane
  • 6: hexane
  • 7: heptane
  • 8: octane
  • 9: nonane
  • 10: decane
• Example: butane ($C<em>4H</em>{10}$).
• Isomers are compounds with the same formula but different structures.
• Structural isomers have the same formula but different groupings branch from the main carbon chain.
• Example: Structural isomers of $C<em>6H</em>{14}$ (hexane).
• Each isomer needs a unique name; they cannot all be called hexane.
• The name 'hexane' is for the normal (straight chain) structure.

Rules for Naming Branched Alkanes:

1. Name the longest continuous carbon chain as the parent name.

2. Identify side groups (alkyl groups) attached to the chain and place them before the parent name in alphabetical order.

   • Alkyl group: an alkane with one hydrogen atom removed.
     • methane ($CH<em>4$) becomes methyl ($CH</em>3$)
     • ethane ($C<em>2H</em>6$) becomes ethyl ($CH<em>3CH</em>2$ or $C<em>2H</em>5$)

3. If several groups of the same kind are attached to the main chain, list the group only once using numerical prefixes (di, tri, tetra, etc.).

4. Assign a number to each side group to indicate its attachment point on the main chain, numbering from the end that gives the lowest set of numbers.

5. Hyphens separate numbers and letters, commas separate numbers.

6. di, tri, tetra, n, s, and t prefixes are not included in alphabetizing, but iso is.

7. The prefix 'cyclo' is used for cyclic alkanes.

   • Examples:
     • 2,4-dimethylhexane
     • 5-ethyl-2,3-dimethylheptane
     • 5-ethyl-3,3-dimethylheptane (not 3-ethyl-5,5-dimethylheptane)

Common Side Groups:

• methyl ($CH_3$)

• ethyl ($CH<em>3CH</em>2$)

• propyl ($CH<em>3CH</em>2CH_2$) (n-propyl)

• isopropyl ($CH(CH<em>3)</em>2$)

• butyl ($CH<em>3CH</em>2CH<em>2CH</em>2$) (n-butyl)

• sec-butyl

• isobutyl ($CH<em>3CH(CH</em>3)CH_2$)

• tert-butyl ($C(CH<em>3)</em>3$)

• phenyl ($C<em>6H</em>5$)

• When identifying the longest chain, the 2-dimensional representation can be misleading.

Alkenes

• Hydrocarbons with at least one carbon-carbon double bond ($C=C$).

1. Select the longest continuous carbon chain containing the double bond as the parent structure. Replace 'ane' with 'ene'.

2. Number the chain to give the carbon atom starting the double bond the lowest number. Prefix the name with this number.

3. Treat side groups as for alkanes.

4. Dienes contain two double bonds, trienes have three, etc.

   • Examples:
     • 1-butene ($CH<em>2=CHCH</em>2CH_3$)
     • 2-butene ($CH<em>3CH=CHCH</em>3$)
     • 7,7-dimethyl-4-propylnon-2-ene
     • 1,3-pentadiene
     • cyclohexene
     • 5-methylcyclopent-1,3-diene

Configurational Isomerism in Alkenes

• Structural isomerism involves different arrangements of carbon atoms.
• Configurational isomerism involves different arrangements in space of atoms in one structural isomer, due to restricted rotation about a double bond.
• If the two highest priority groups (left and right side of double bond) are on the same side of the double bond (top or bottom) then the double bond has the Z configuration (zusammen, German for together) and if they are on opposite sides the configuration is E (entgegen, German for opposite).
• (Z)-2-butene: highest priority groups on the same side.
• (E)-2-butene: highest priority groups on opposite sides.
• 2-methyl-1-propene: a different structural isomer.

Alkynes

• Hydrocarbons with at least one carbon-carbon triple bond ($C≡C$).

1. Nomenclature and numbering are the same as alkenes, except replace 'ene' with 'yne'.

2. Do not exhibit configurational isomerism due to the linear nature of the triple bond.

   • Examples:
     • ethyne (acetylene) ($HC≡CH$)
     • 6,6-diethyl-3-octyne

Aromatic Hydrocarbons

• Based on the presence of benzene-like rings.

  • Examples:
    • benzene
    • naphthalene
    • anthracene

• Substituents are treated as in cyclic alkanes.

  • Examples:
    • methylbenzene (toluene)
    • 1,3-diethyl-2-methylbenzene

Alkyl Halides

• Halogens (F, Cl, Br, I) are treated as fluoro, chloro, bromo, and iodo side groups in nomenclature.

  • Examples:
    • 2-chloro-2-fluoro-3,3-dimethylhexane
    • 1,1-diiodo-2-methylprop-1-ene

Functional Groups

• Common classes of organic compounds containing oxygen and nitrogen.

• Functional Groups:

  • Alcohol: $C-OH$
  • Ether: $C-O-C$
  • Carboxylic acid: $C(=O)OH$
  • Aldehyde: $C(=O)H$
  • Ketone: $C(=O)C$
  • Amine: $C-N$
  • Amide: $C(=O)N$
  • Ester: $C(=O)OC$

Alcohols

• Contain a hydroxyl group (-OH) bonded to an $sp^3$ carbon atom.

1. Select the longest continuous carbon chain containing the carbon attached to the alcohol group. Replace 'ane' with 'anol'.
2. Number the chain to give the -OH group the lowest number. Prefix the name with this number.
3. Treat side groups as before.

• Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of alkyl groups bonded to the carbon attached to the -OH group.

  • Examples:
    • pentan-2-ol
    • 1,1,1-trichloro-3,3-dimethylhexan-2-ol
    • (Z)-1,2-cyclohexanediol

Ethers

• Contain an oxygen bridge (R-O-R').

• Treat the shortest (R-O-) group as an alkoxy side group on the longest carbon chain.

  • Examples:
    • 1-methoxyethane (ethyl methyl ether)
    • 4-ethoxy-2-methylhexane

Aldehydes

• Contain a carbonyl group at the end of a carbon chain $\begin{matrix} & O \ & || \ C & -H \end{matrix}$.

1. Select the longest continuous carbon chain containing the terminal aldehyde group. Replace 'ane' with 'anal'.

2. Number the chain such that the aldehyde carbon is atom number one.

3. Treat side groups as before.

4. The number '1' for the 'al' group need not be included in the name.

   • Examples:
     • ethanal
     • methanal (formaldehyde)
     • 3-chloro-3-methylbutanal

Ketones

• Contain a non-terminal carbonyl group ($C=O$).

1. Select the longest continuous carbon chain containing the carbonyl group carbon atom. Replace 'ane' with 'anone'.

2. Number the chain such that the carbonyl carbon has the lowest number. Prefix the name with this number.

3. Treat side groups as before.

   • Examples:
     • 2-pentanone
     • 3,4-dibromobutan-2-one
     • cyclohexanone

Organic Acids

• Contain the carboxylic acid group ($COOH$ or $CO_2H$).

1. Select the longest continuous carbon chain containing the carboxylic acid group. Replace 'ane' with 'anoic acid'.

2. Number the chain such that the carboxylic acid carbon is atom number one.

3. Treat side groups as before.

4. The number '1' for the acid group need not be included in the name.

   • Examples:
     • methanoic acid (formic acid)
     • 2,2,2-trifluoroethanoic acid
     • 1,2-ethanedioic acid (oxalic acid)

Esters

• Carboxylic acid derivative containing R-C(=O)-OR', where R and R' are alkyl or aryl groups.

1. Locate the carboxylic acid portion and name it as the parent carboxylic acid. Replace 'anoic acid' with 'oate'. Name the alcohol portion as a radical, placed in front of the parent name.

2. Carbon atom number '1' is the ester carbonyl.

   • Examples:
     • methyl ethanoate
     • isopropyl 2,2-dimethylbutanoate

Amines

• Contain nitrogen attached to $sp^3$ carbon atoms ($N$).

1. Primary Amines:

   • Named either by:

     • Adding the suffix amine to the name of the alkyl group bonded to the nitrogen atom, written as one word.

     • Replacing the final e of the IUPAC name of the parent alkane with amine.

     • Examples:

       • isopropylamine
       • sec-butylamine
       • cyclohexylamine
       • 2-methyl-2-ethanamine
       • 2-butanamine
       • cyclohexanamine

2. Secondary and Tertiary Amines:

   • Symmetrical amines are named by adding the prefix di or tri to the name of the alkyl group.

     • Examples:
       • diethylamine
       • trimethylamine
       • triphenylamine

   • Unsymmetrically substituted amines are named as N-substituted primary amines. The largest alkyl substituent is the parent chain. The letter N indicates that alkyl groups are attached to the nitrogen atom.

     • Examples:
       • N-methylpropan-1-amine
       • N-ethyl-N-methylbutan-1-amine
       • N,N-dimethylbutan-2-amine

   • The –NH2 group in complicated structures is called an amino group and is treated as any other substituents.

     • Examples:
       • 3-aminocyclohexanone
       • (4E)-3-aminohex-4-en-2-ol
       • ethyl 3-aminobutanoate

Amides

• Contain the group $\begin{matrix} & O \ & || \ C & -N \end{matrix}$.

• Named like esters: treat as a carboxylic acid derivative using the 'amide' ending. Name the amine portion as a radical placed in front of the parent name. No space between the alkyl group and the parent name. N-alkyl groups are listed alphabetically.

  • Examples:
    • N-methylpropanamide
    • N,N-dimethylpropanamide
    • (2E)-N-isopropyl-N-methylbut-2-enamide

Aromatics

• Various functional groups may be present in aromatic compounds. Common names are often used.

  • Examples:
    • toluene
    • phenol
    • benzaldehyde
    • benzoic acid
    • aniline
    • 2-naphthol
    • 2,4,6-trinitrotoluene (T.N.T.)
    • 2,4-dichlorophenoxyacetic acid (2,4-D)
    • a polychlorinated biphenyl (PCB)

Compounds with More Than One Functional Group

• Classified by the principle group (highest on the hierarchy scale).

  • Hierarchy: carboxylic acid > aldehyde > ketone > alcohol > amine > alkyne = alkene > alkane.

• The parent name is derived from the principle group.

• An alcohol and aldehyde is named as an aldehyde with an alcohol side group.

• An alcohol, ketone, and acid is named as an acid with alcohol and ketone side groups.

• The numbering system is that for the principle group.

• An alcohol is regarded as a hydroxy-side group, an amine as an amino-side group, and a carbonyl as an oxo-side group.

  • Examples:
    • 2-amino-3-buten-1-ol
    • 1,3-dihydroxypent-4-yn-2-one
    • 4-oxohexanoic acid
    • 2-amino-3-hydroxy-5-oxo-8-phenylhept-2-en-6-ynoic acid

Configurational Isomerism Revisited - Optical Isomerism

• An sp3 carbon atom that is asymmetric (bonded to four different groups) displays configurational isomerism.
• Such carbon atoms are called chiral.
• The molecule and its mirror image (isomer) are non-superimposable.
• Isomers twist a beam of monochromatic polarized light either to the right or left (dextro and laevo).
• A different naming system, analogous to the E & Z system for alkenes, has replaced the d and l labels.
• i.e., d-lactic acid and l-lactic acid.