Unit 3: Section 3- Alkenes and Alcohols + RP5 Flashcards
What is the general formula of alkenes?
CnH2n
How can you use one word to describe alkenes?
unsaturated
What shape is the arrangement of bonds around the C=C bond?
planar
What is the bond angle in alkenes around the C=C bond?
120
Explain why alkenes are vulnerable to attack by other species
Because the pi bonds are exposed which have high electron density.
What does a C=C double covalent bond consist of?
one sigma bond
one pi bond
What type of isomerism can alkenes exhibit?
E-Z stereoisomerism
Define electrophile
an electron pair acceptor
What is an addition reaction?
where two reactants form one product
Describe each step in the electrophilic addition reaction between bromine and alkenes
- the C=C bond repels the electrons in Br2, polarising Br-Br
- the double bond then attracts the Br slightly +, and forms a bond with it. this repels electrons in the Br-Br bond further, until it breaks
- you get a positively charged carbocation intermediate, the Br slightly - bonds to the C
- one product is formed
What is a carbocation?
a positively charged carbon atom
What do alkenes form when they react with hydrogen halides?
halogenoalkanes
Give three examples of electrophiles
H+
NO2+
Br slightly +
Draw the mechanism for the electrophilic addition reaction between ethene and hydrogen bromide
Why is HBr a polar molecule?
Br is more electronegative than H
What happens if you add a hydrogen halide to an unsymmetrical alkene?
two isomeric products- 1 major product, 1 minor product
What does the amount of product formed in electrophilic addition of alkenes and hydrogen halides depend upon?
how stable the carbocation intermediate is
What is the order for stability of carbocations?
tertiary (most)
primary (least)
Draw the mechanism for hydrogen bromide reacting with propene
What is the chemical formula for sulphuric acid?
H2SO4
What are the two changes in functional groups in electrophilic reactions with alkenes and H2SO4?
- change in functional group from alkene to alkyl hydrogen sulphate
- change in functional group from alkyl hydrogen sulphate to alcohol
Describe the reaction between sulphuric acid and alkenes
- cold concentrated sulphuric acid reacts with an alkene in an electrophilic addition reaction
- if you then add cold water, and warm the product, it hydrolyses to form an alcohol
- the sulphuric acid isn’t used up- it acts as a catalyst
How can the bromine water test be used quantitively?
to show the presence of multiple bonds in compounds like polyunsaturated oils
What is addition polymerisation?
when addition polymers are formed from alkenes
What is a polymer?
lots of monomers joined together
What is one word to describe polyalkene chains?
saturated
What does saturated mean?
they only contain single carbon bonds in the carbon chain
How does increasing length of polyalkenes effect Van der Waals forces?
the longer the polymer chains are, the closer together they can get, so the stronger the Van der Waals forces between the chains will be
Describe the differences in physical properties of short chain and long chain polyalkenes
short, branched chains- weaker and more flexible
long, straight chains- strong and rigid
How can you make PVC more flexible?
adding a plasticiser
What is rigid PVC used for?
drainpipes and window frames
What can plasticised PVC be used for?
electric cable insulation, clothing, flooring tiles
What is the general formula of alcohols?
CnH2n+1OH
What is the H-C-H and C-C-O bond angle and the shape?
109.5
tetrahedral
Explain the H-C-H and C-C-O bond angle
109.5
because there are 4
bonding pairs of electrons
repelling to a position of
minimum repulsion
What is the H-O-C bond angle and the shape?
104.5
bent line shape
Explain the H-O-C bond angle?
104.5
there are 2 bonding
pairs of electrons and 2 lone pairs
repelling to a position of minimum
repulsion. Lone pairs repel more than
bonding pairs so the bond angle is
reduced.
Explain the boiling points of alcohols
the alcohols have relatively high boiling points due to their ability to form hydrogen bond between alcohol molecules
Describe what happens when smaller alcohols react in water
the can dissolve in water because they can form hydrogen bonds to water molecules
Describe a primary alcohol
1 carbon is attached to the carbon adjoining the oxygen
Describe a secondary alcohol
2 carbons are attached to the carbon adjoining the oxygen
Describe a tertiary alcohol
3 carbon are attached to
the carbon adjoining the oxygen
What is potassium dichromate?
oxidising agent
What reaction occurs when primary alcohols are partially oxized?
the primary alcohol forms an aldehyde and water
What is an aldehyde?
contains the functional group with the structure R-CH=O
What are the reagents in the partial oxidation of a primary alcohol?
potassium dichromate (VI) solution and dilute sulfuric acid
Describe the conditions needed for the partial oxidation of primary alcohols
(use a limited amount of dichromate) warm
gently and distil out the aldehyde
Why should you use the approximate boiling point of the desired aldehyde and not higher when distilling?
in order to maximise the yield collected
Where does the water go in and out in a condenser?
water goes in at the bottom to go against gravity, so prevents backflow
Give a reason why some organic chemicals are heated with an electric heater
organic chemicals are normally highly flammable and could set on fire with a naked flame
Describe the reaction of the full oxidation of primary alcohols
Reaction: primary alcohol -> carboxylic acid
Reagent: potassium dichromate(VI) solution and dilute
sulfuric acid
Conditions: use an excess of dichromate, and heat
under reflux: (distil off product after the reaction has finished)
observation- the orange dichromate ion reduces to the green Cr3+ ion
When is reflux used?
when heating organic reaction mixtures for long periods
Why are anti-bumping granules used?
to prevent vigorous/ uneven boiling
What is a ketone?
has two alkyl groups attached to the carbonyl carbon atom
