[14.1] properties of alcohols

Question 1

what is the general formula for the homologous series of alcohols?

Answer 1

CₙH₂ₙ₊₁OH

Question 2

classification of alcohols: aliphatic

Answer 2

• provided there are no rings
• OH replaces H in a basic hydrocarbon skeleton
• OH is written separately in skeletal formula to emphasise that it is the functional group
• /\/\ is butan-1-ol

Question 3

classification of alcohols: aromatic

Answer 3

• in aromatic alcohols (or phenols) the OH is attached directly to the ring
• an OH on a side chain of a ring behaves as a typical aliphatic alcohol

Question 4

structural differences between aliphatic and aromatic alcohols

Answer 4

• alcohols are classified according to the environment of the OH group
• chemical behaviour eg. oxidation often depends on the structural type

Question 5

primary, secondary and tertiary alcohols

Answer 5

• draw them!
• primary: -OH is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group
• secondary: -OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups
• tertiary: -OH group is attached to a carbon atom that is attached to no hydrogen atoms and three alkyl groups

Question 6

naming alcohols according to standard IUPAC rules

Answer 6

• select the longest chain of C atoms containing the O-H group
• remove the e and add ol after the basic name
• number the chain starting from the end nearer to the O-H group
• the number is placed after the an and before the ol eg. butan-2-ol
• as in alkanes, prefix with alkyl substituents
• side chain positions are based on the number allocated to the O-H group
• eg. CH₃-CH(CH₃)-CH₂-CH₂-CH(OH)-CH₃ is called 5-methylhexan-2-ol

Question 7

in what ways are structural isomers possible in alcohols?

Answer 7

• different positions for the OH group
• branching of the carbon chain

Question 8

what are the two ways that ethanol can be made?

Answer 8

• fermentation of carbohydrates
• reaction of ethene with steam

Question 9

details about fermentation of carbohydrates

Answer 9

• glucose -> ethanol + carbon dioxide (i’m sure you can manage to write the equation)
• conditions: 30-40°C, yeast catalyst, anaerobic conditions
• slower, produces impure ethanol

Question 10

details about reaction of ethene with steam

Answer 10

• ethene + steam -> ethanol (i’m sure you can manage to write the equation)
• conditions: 300°C, 60 atm, solid phosphoric (V) acid catalyst
• faster, produces pure ethanol

Question 11

compare the physical properties of alcohols compared to their corresponding alkanes

Answer 11

• less volatile
• have higher melting points
• greater water solubility
• the differences become much smaller as the length of the carbon chain increases

Question 12

explain trends in boiling points for alcohols compared to alkanes and as carbon chain length increases

Answer 12

• bp of alcohols are higher than alkanes because there is hydrogen bonding due to the OH group
• hydrogen bonding is stronger and so requires more energy to overcome
• boiling points of alcohols increases with increasing number of carbons because there is an increase in the strength of the london forces

Question 13

explain the solubility in water of alkanes and alcohols

Answer 13

• alkanes are not soluble in water because they are non polar and therefore cannot form hydrogen bonds with water
• short chain alcohols are soluble in water because they have a polar OH group which can form hydrogen bonds with water molecules
• however, as the hydrocarbon chain increases, the influence of the OH group becomes relatively smaller and the solubility decreases

Question 14

describe the polarity of the bonds in alkanes and alcohols

Answer 14

• alkanes have non-polar bonds because the electronegativity of hydrogen and carbon are very similar
• alkane molecules are therefore non-polar
• intermolecular forces between non-polar molecules are very weak london forces
• alcohols have a polar O-H bond because of the difference in electronegativity of the oxygen and hydrogen atoms
• alcohol molecules are therefore polar
• the intermolecular forces will be very weak london forces but there will also be much stronger hydrogen bonds between the polar O-H groups (eg. in two ethanol molecules)

Question 15

what does volatility mean?

Answer 15

• a measure of how readily a substance vaporises
• volatile refers to a substance that vaporises readily

Question 16

how does the volatility of alcohols change with boiling point compared to their corresponding alkanes?

Answer 16

volatility increases as boiling point decreases