Topic 10.2 - Functional groups Flashcards
Alkanes
Aliphatic (Single chain), simplest hydrocarbons, volatile (readily changes chemical state). relatively inert, low bond polarity, high covalent bond strength, used as fuel (due to the energy released during combustion).
As length increases: volatility decreases, melting and boiling point increase
Reactions: combustion, halogenation
Combustion of alkanes
Produces: carbon dioxide and water
Conditions: excess oxygen
Alkane + oxygen -> carbon monoxide + water
Incomplete combustion of alkanes
Produces: carbon monoxide and water
Conditions: not enough oxygen
Alkane + oxygen -> carbon monoxide + water
Halogenation of alkanes
Produces: halogenoalkanes
Conditions: ultraviolet light
Free radical substitution - nucleophilic substitution
Free radical substitution
Initiation -> reaction of ultraviolet light with a halogen compound (HX or X₂). Free radicals produced as homolytic fission occurs.
X₂ -> X• + •X
HX -> H + •X
Propagation -> free radicals generated more as further reactions occur.
(1) Halogen radical reacts with alkane, forming a radicle from the alkane (•CₙH₂ₙ₋₁)
X• + CₙH₂ₙ -> •CₙH₂ₙ₋₁
(2) alkane radicle reacts with halogen to form more halogen radicles₂
X₂ + •CₙH₂ₙ₋₁ -> CₙH₂ₙ + X•
Termination -> ends with the radicals being removed as they join up together. Usually occurs when the concentration of hydrocarbons starts to decrease.
Cl• + •Cl -> Cl₂ H₃C• + •CH₃ -> C₂H₆
H• + •Cl -> HCl
Alkenes
Single chain (simplest hydrocarbons), low bond polarity, high covalent bond strength, relatively inert.
Physical properties:
Chemical properties:
Reactions: combustion, halogenation
Finish
Halogenation of alkenes
Produces: halogenoalkanes
Conditions: halogen halide/halogen₂
Alkene + halogen halide -> x-halogenoalkane
Alkane + halogen₂ -> x,x-halogenoalkane
Hydration of alkenes (H)
Produces: alkanes
Conditions: 150°C, Nickel,
alkene + hydrogen₂ -> alkane
Alkenes to alcohol
Produces: Alcohols
Mechanism: Hydration
Conditions: 300°C, phosphoric (v) acid, 6-7 MPa
alkene + steam -> alcohol
Alcohols
Contain an -OH bond, variety of uses (ie biofuel), can be oxidised with potassium manganate (VII) (KMnO₄) and/or potassium dichromate (VI) (K₂Cr₂O₇) to produce ketones, aldehydes, and carboxylic acid.
Combustion of alcohols
Produces: carbon dioxide and water
Conditions: oxygen
Alcohol + oxygen -> carbon dioxide + water
Oxidation of primary alcohols
Produces: aldehyde then carboxylic acids
Conditions: acidified potassium dichromate (VI) (K₂Cr₂O₇)
Alcohol + [O] -> aldehyde
Further refluxing leads to the formation of carboxylic acids as further oxidation occurs (without refluxing the aldehyde would leave as an aldehyde):
Aldehyde + [O] -> carboxylic acid
Oxidation of secondary alcohols
Produces: ketones
Conditions: acidified potassium dichromate (VI) (K₂Cr₂O₇)
Alcohol + [O] -> ketone
Condensation reaction of carboxylic acid and alcohol
Produces: esters
Conditions: concentrated sulfuric acid
Alcohol + carboxylic acid -> esters
Testing for alkenes
Using halogens, alkenes can be tested
Adding halogens, for example, bromine, the solution will turn the colour of the halogen. This will then turn colourless if alkenes are present.
One bromine will be more δ⁺ than the other due to an induced dipole. This will cause the δ⁺ bromine to react with the pi bond and form a bond with the carbon. The electrons from the halogen bond will be given to the δ⁺ bromine. The other carbon in the pi bond would become a carbonation. This carbonation would then react with the Br⁻ and this halogen attaches too, leading to the formation fo an x,x-halogenoalkane.
