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SolutionsStructure and Nomenclature

1. General formula

Write the molecular formula of the alkanes with 5, 8 and 20 carbon atoms. How many hydrogen atoms does an alkane with n carbons have?

C5H12, C8H18 and C20H42. An acyclic alkane has 2n + 2 hydrogens: CnH2n+2.

2. Isomers of pentane

Draw and name all the isomers of formula C5H12.

  1. Pentane: CH3CH2CH2CH2CH3
  2. 2-Methylbutane: (CH3)2CHCH2CH3
  3. 2,2-Dimethylpropane: C(CH3)4

They are three chain isomers (constitutional isomers).

3. Types of carbon

In 2,2,4-trimethylpentane, (CH3)3CCH2CH(CH3)2, state how many primary, secondary, tertiary and quaternary carbons there are.

Primary: 5 (the five CH3 groups); secondary: 1 (the CH2); tertiary: 1 (the CH); quaternary: 1 (the C bonded to three methyls and the CH2).

4. Name these alkanes

  1. CH3CH(CH3)CH2CH3
  2. CH3CH2CH(CH2CH3)CH2CH3
  3. (CH3)2CHCH(CH3)2
  4. CH3CH2CH2CH(CH3)CH2CH2CH3
  1. 2-Methylbutane
  2. 3-Ethylpentane
  3. 2,3-Dimethylbutane
  4. 4-Methylheptane

5. Is there a stereocentre?

Which of these alkanes have a stereocentre?

  1. 3-Methylpentane
  2. 3-Methylhexane
  3. 2-Methylbutane
  4. 2,3-Dimethylpentane
  1. No: C3 is bonded to two identical ethyl groups.
  2. Yes: C3 is bonded to H, CH3, ethyl and propyl, four different groups.
  3. No: C2 bears two methyl groups.
  4. Yes: C3 (bonded to H, CH3, ethyl and isopropyl). C2 is not, because it bears two methyls.

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