Thermochemistry 2.1

• Hydrocarbons


  Non-polar, covalent compounds with only weak van der Waals forces of attraction between the molecules
• The longer the chain length

  The higher the boiling point
• Functional group


  The atom or group of atoms in a compound that gives the compound its characteristic properties
• Homologous series


  A 'family' of compounds that have the same functional group. They can be represented by a general formula, e.g. alkanes, CnH2n + 2. They differ from their neighbour in the series by one CH2 unit. Having the same functional group means that they have similar chemical properties. Their physical properties vary with a general trend as the Mr varies.
• Hydrocarbon


  A compound containing hydrogen and carbon only
• Prefixes for number of carbon atoms

  • meth- (1)
  • eth- (2)
  • prop- (3)
  • but- (4)
  • pent- (5)
  • hex- (6)
  • hept- (7)
  • oct- (8)
• Rules for naming organic compounds

  1. Identify the compound's functional group/type

  2. Find the longest continuous carbon chain that contains the functional group (be careful as branches can be misleading)

  3. Number the carbon atoms in the longest chain, starting from the end that gives any side chains or functional groups the smallest numbers possible

  4. Name any branched groups/substituted groups and give their position on the carbon chain (e.g. 3). If there is more than one of the same group attached, then use the prefix di- or tri-. If there is more than one type of group attached, they are written in alphabetical order

  5. A —CH3 group is called methyl, a —CH2CH3 group is called ethyl, etc.

  6. Combine the parts of the name into one word; branched groups, prefix, name ending (suffix). If there are two consecutive numbers, a comma is placed between them. A dash separates words from numbers
• Naming example


  • 3-methylhexane
• Alkenes


  Functional group: C=C (they are unsaturated hydrocarbons)
• Naming example


  • 2-ethylbut-1,3-diene
• Halogenoalkanes


  Functional groups: —F (fluoro-), —Cl (chloro-), —Br (bromo-), —I (iodo-)
• Naming example


  • 1,3-dibromo-3-fluorobutane
• Alcohols


  Functional group: —OH (-ol)
• Naming example


  • butan-1,2-diol
• Carboxylic acids


  Functional group: —COOH (-oic acid)
• Naming example


  • 2-ethylhexanoic acid
• Molecular formula


  The number of atoms of each element present in the molecule
• Shortened formula


  Shows some detail about the structure and the functional group without including bonds
• Displayed formula


  Shows all the atoms and the bonds in the molecule
• Skeletal formula


  Shows the bonds between any carbon atoms in the molecule and any functional group that is attached
• Structural isomerism


  Compounds with the same molecular formula but a different structural formula
• Chain isomerism


  • The carbon chain is arranged differently (branches). A branched isomer has a lower boiling temperature as the chains are further away from one another (reduction in van der Waals forces).
• Chain isomerism example


  • butane, methylpropane
• Position isomerism


  • The functional group is in a different position in the molecule.
• Position isomerism example


  • 1-chloropropane, 2-chloropropane
• Functional group isomerism


  • The functional group is different, e.g. as in aldehydes and ketones.
• Functional group isomerism example


  • propanal, propanone
• 1. Z isomerism


  • Only occurs in alkenes because there is restricted rotation about the double bond.
• 1. Z isomerism example


  • (E) - 1,2-dichloroethane, (Z) - 1,2-dichloroethane
• Determining E or Z configuration

  Look at the atoms directly attached to each of the carbon atoms. The atom with the higher atomic number takes priority. If the higher priority atom on both carbon atoms is on the same side of the double bond, the isomer is the Z form. If the higher priority atoms are on opposite sides of the double bond, the isomer is the E form.
• 1. Z isomerism example


  • F-C=C-I, Br-C=C-Cl
• Properties of E-Z isomers


  • Both isomers undergo the characteristic reactions of alkenes. However, the restricted C=C rotation means that the substituent groups can behave differently. In the E form, the substituent groups are further apart than they are in the Z form. For example, in butenedioic acid, the two —COOH groups in the Z form can interact in a dehydration reaction. This does not happen with the E form as the groups are too far apart.
• Structural formula examples


  • Molecular formula: C3H6O2

  Shortened formula: CH3CH2COOH

  Displayed formula: H-C-C-C-O-H

  Skeletal formula: H3C-CH2-COOH