UNit 10- Alkenes and alcohols

Question 1

Alkenes and double bonds

Answer 1

• Unsaturated, contain C=C bonds (612KJmol^-1) and C-C bonds (347KJmol^-1)
• general formula CnH2n
• double bonds contain pi-bonds and sigma bonds
• sigma bonds are end to end overlaps of orbitals and represent single bonds
• Pi bonds are weaker and make sigma bonds into double bonds by horizontally overlapping above and below a sigma bond
• Pi bonds have two orbitals, and as such a lower electron density
• they also prevent rotation around the sigma bond, creating E/Z isomerism

Question 2

Electrophilic addition- saturation of alkenes

Answer 2

• double bond can be easily attacked by electrophiles
• in some cases (e.g. Bromine test), an electrophile takes electrons from the double bond to bond to one of the Carbons
• the other part of the electrophilic molecule is then negative, and is therefore attracted to the carbocation formed on the other side of the double bond

Question 3

Electrophilic addition- production of alcohol using H2SO4

Answer 3

• an electrophile is formed from the dipole between and H-O bond in the H2SO4
• The H then takes an electron pair from the double bond and bonds itself to one of the carbons, while the rest of the HSO4- bonds to the carbocation
• heating this with water leads to the rest of the molecule reducing away to form an -OH, creating an alcohol

Question 4

Electrophilic addition- production of halogenoalkanes using hydrogen halides

Answer 4

• similar to saturation only the electrophile is always a hydrogen, and there is always a halogen to bond to the carbocation
• in asymmetrical alkenes the carbocation could form either side of the double bond depending on how many adjacent carbon atoms there are around it
• the carbocation will be more stable when surrounded by more carbons, and this will alter the likeliness of different halogenoalkanes to be formed (although all possible ones will form)

Question 5

Addition polymers

Answer 5

• many monomers bonded together n amount of times
• double bond opens up to bond to other alkenes and form a potentially never ending chain of alkenes
• -can also be halogenoalkanes, alkenes with alkyl groups etc.
• longer strands have stronger IMF, stands with less branches will have stronger IMF
• plasticisers push chains apart to ‘soften’ plastics
• addition polymers are unreactive as they have no polar bonds
• PVC, Poly(chloroethane), is used to make drain pipes and window frames as it is brittle

Question 6

Alcohol production

Answer 6

• Ethanol can be made by industrial hydration of ethene, which has a yield of only 5%, but products can be recycled to get a yield of 95%
• also works the other way around e.g dehydration of alcohol into alkenes using conc H2SO4 as a catalyst (elimination reaction- this is a source for monomers that is more sustainable)
• ethene for ethanol can come from cracking/ distillation of oils
• ethanol can come from fermentation of glucose with yeast, which will die after producing 15% conc of alcohol, and can be denatured at high temperatures
• ethanol from this renewable source is used as a biofuel (once it is separated out by fractional distillation), and is carbon neutral as 6mol carbon goes into glucose during photosynthesis, 2mol come out in fermentation and 4mol come out in combustion
• crops required for fermentation take land for food crops, cause deforestation, and cause pollution from harvesting machinery

Question 7

Oxidation of alcohols

Answer 7

• Aldehydes are produced from 1st oxidation of primary alcohols
• ketones are produced from 1st oxidation of secondary alcohols
• carboxylic acids are produced from 2nd oxidation of primary alcohols
• tertiary alcohols cannot be oxidised
• a suitable oxidising agent is acidifed potassium dichromate (vii)
• oxidation to aldehydes requires hot, ethanolic conditions with H2SO4 and [O] with distilling apparatus
• oxidation to ketones requires reflux, acidic conditions and [O]
• oxidation to carboxylic acids requires reflux, ethanolic conditions and an excess of [O]
• Fehling’s and Benedict’s solutions will turn from blue to brick-red with aldehydes but not ketones
• Tollen’s reagent reduces from colourless to from a silver mirror on the test tubes surface with an aldehyde but not a ketone