alkenes

Question 1

Describe alkenes

Answer 1

• unsaturated which means there is at least 1 = covalent bond
• can undergo addition reactions
• = have a high e- density making them reactive

Question 2

what are sigma bonds

Answer 2

• are when 2s orbitals overlap
• align horizontally to give a single covalent bond
• strong electrostatic attraction between the nuclei of the shared pair of electrons (bc of high electron density between the nuclei)
• high bond enthalpy
• strongest type of covalent bond

Question 3

what are pi bonds

Answer 3

• sideways overlap of 2 p orbitals above and below the C=C.
• PB are weaker than SB bc the e- density is spread out above and below the nuclei
• electrostatic attraction is weaker so SB have a lower bond enthalpy
• PB locks the 2 C-atoms in position and prevents them from rotating around the =, so rotation is restricted.

Question 4

describe the reactivity of alkenes

Answer 4

• alkenes have SB + PB
• 3/4 used in SB.
• this is a high electron density area that makes them reactive
• PB stick out a little and the = has a high ED
• open to attack from electrophiles
• PB has a low bond enthalpy so alkenes are reactive bc not a lot of energy needed

Question 5

shape of alkenes

Answer 5

• trigonal planar
• three regions of e- density around each carbon atom
• three regions repel AFAP
• bond angle is 120

Question 6

stereoisomers

Answer 6

• same structural formula
• different arrangement of atoms in space
• e/z isomerism occurs when there is restricted rotation around = and requires 2 diff FG attached to each c-atom
• E + trans= opposite
• Z + cis- same side

Question 7

describe the Cahn Inlog Prelog rules

Answer 7

• when there are 4 diff groups around a double bond
• priority based on their atomic number
• calculate the atomic number of the first element directly bonded to C=C.
• if first element is the same number, move down the chain .

Question 8

describe test for alkenes

Answer 8

• decolourisation of Br water
• adding Br water to alkenes causes a colour change from brown/orange to colourless
• Br is the electrophile and adds to the alkene forming dibromo alkane
• Br2 is polarised as the e- in the = repels the e- in Br2
• Br(-)-(Br(+)
• Br2 bond breaks by heterolytic fission
• double bond breaks
• one carbon bonds to a Br
• carbocation intermediate is formed and a Br- with 2 electrons
• addition product formed

Question 9

what is an electrophile

Answer 9

• electron pair acceptor
• deficient in electrons so are attracted to a =
• H+ , NO2+
• E.g polar molecules (delta+) H-Br

Question 10

Hydration of alkenes

Answer 10

• alcohol is formed
• sue steam and acid catalyst to create and alcohol
• can make ethanol by reacting steam and ethene using a phosphoric acid catalyst + 300C + 60atm

Question 11

addition of hydrogen halides to alkenes

Answer 11

• electrophilic addition

- halogenoalkanes formed,

Question 12

reaction of alkenes with unsymmetrical hydrogen halide

Answer 12

• 2 different products are formed
• the more stable carbocation forms the major product
• the less stable carbocation forms the minor product

Question 13

what determines the stability of a carbocation

Answer 13

• the more alkyl groups bonded to carbocation the more stable
• bc alkyl groups push e- towards the +ve carbocation stabilising it

Question 14

describe addition polymers

Answer 14

• alkenes are the monomers which join to form addition polymers
• natural : proteins and natural rubber
• synthetic : poly(ethene) and poly(propene)

Question 15

how to make poly(propene)

Answer 15

• need several monomers of propene
• the double bond opens up to form the polymer
• the monomer will have a bracket around it
• each carbon will have no = and a single bond coming out of the bracket to form 4 bonds
• poly(alkanes) are saturated, normally non-polar and unreactive.
• they don’t degrade well in landfill.

Question 16

positives of landfill in the disposal of plastic

Answer 16

• most polymers aren’t biodegradable so need to be disposed of carefully
• landfill is useful for disposing of plastics that are :
• too difficult to recycle
• too difficult to separate from other materials
• ## not enough plastic to extract to make in economically viable

Question 17

negatives of landfill in the disposal of plastics

Answer 17

• not sustainable as large amounts of land needed
• increasingly expensive to use land for waste disposal (land tax)
• there is a need to reduce our reliance on landfill.

Question 18

recycling plastics

Answer 18

• most plastics made from crude oil (non renewable)
• recycling means we reduce reliance on crude oil making plastics
• some plastics (polypropene) can be remoulded into new objects
• other plastics can be cracked into monomers to be used as organic feedback for plastics or other substances.

Question 19

incineration

Answer 19

• burning waste plastics
• if can’t be recycled
• energy from burning can be used to generate electricity
• can release toxic fumes
• chlorine based plastic s(PVC) can produce harmful HCL gas when burnt
• glue gas scrubbers neutralise acidic gases (HCL). fire a base at the acidic gas to form a neutral salt.

Question 20

biodegradable polymers

Answer 20

• decompose under certain condition by organisms
• made from oil fractions and renewable sources (starch)
• more expensive
• to degrade you need good supply of O2 and moisture
• can use BP in frost protective sheets for plants. made form polyethene and starch grains. over time micro-organisms break down the polymer so you don’t need to dispose of the old sheeting.
• photodegradable polymers degrade when exposed to sunlight.