alkenes

Question 1

what is the general formula for an alkene

Answer 1

CnH2n

Question 2

are alkenes unsaturated or saturated

Answer 2

unsaturated as contain the c=c

Question 3

explain the planar structure of alkenes

Answer 3

ethene is completely planar but in larger alkenes it’s only the c=c unit which is planar, the four bonds coming off are 3D

Question 4

explain rotation around the c=c bond in alkenes

Answer 4

no rotation around the double bond

two covalent bonds between the two C atoms

• Bond 1 is sigma, the electrons are between the 2 C atoms
• Bond 2 is pie, electrons above and below the plane of the 6 central atoms from the overlap of P orbitals on the C atom

Question 5

are alkenes more reactive than alkanes and why

Answer 5

more reacive

the double bond with 2 pairs of electrons is a high region of electron density which acts as beacon to anything that needs electrons making it reactive

Question 6

what is the definition of E-Z stereoisomers (geometric)

Answer 6

isomers with the same structural formula but the bonds are arranged differently in space

to have stereoisomerism there must be no rotation and 2 different groups on the carbon

Question 7

explain how stereoismerism works and how to name

Answer 7

isomers which are not mirror image = diastereroisomers

use CIP notation to determine which is the priority group, each bonded atom is prioritised according to its atomic mass, if it doesnt differentiate the next atom in the chain is used etc, dont use the hydrogens to determine

Z = zame side

E = opposite

Question 8

what is an electrophile

Answer 8

electron pair acceptor (opposite to nucleophile)

eg. H⁺ and NO₂⁺

Question 9

explain how alkenes react by electrophilic addition reactions

Answer 9

the double bond in an alkene opens up and atoms are added to the carbon atoms

these reactions happen as the double bond has got plenty of electrons and is easily attacked by electrophiles

general equation:

CH₂=CH₂ + X—Y → CH₂XCH₂Y

Question 10

what is a carbo cation

Answer 10

an organic ion containing a pos. charged carbon

Question 11

show the general mechanism for electrophilic addition reactions

Answer 11

Question 12

show the mechanism between ethene and bromine

Answer 12

equation: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

Question 13

whats the test for unsaturation

Answer 13

use bromine water Br (aq)

the C=C goes to colourless (decolourises)

neg test stays orange

Question 14

show the equation and mechanism of the first step of ethene and sulfuric acid

Answer 14

step one: CH₂=CH₂ + H₂SO₄ → CH₃CH₂OSO₂OH

Question 15

show the equation and mechanism of the second step of ethene and sulfuric acid

Answer 15

hydrolysis = chemical splitting using water in this case to form an alcohol

the sulfuric acid acts as a catalyst as it is regenerated

Question 16

explain reactions with alkenes and hydrogen bromide

example of ethene and hydrogen bromide

Answer 16

form bromoalkanes

example of ethene and hydrogen bromide:

CH₂=CH₂ + HBr -> CH₃CH₂Br

Question 17

explain Markovnikov’s rule

Answer 17

When HBr reacts with an unsymmetrical alkene, the H atom attaches to the C attached to the lower n. of alkyl groups, the Br attaches to the C attached to the most alkyl groups forming the major product.

Question 18

explain the issue with electrophilic addition with unsymmetrical alkenes

Answer 18

for example ethene is a symmetrical alkene, doesn’t matter for eg. with HBr where the Br goes as the product is always bromoethane

Isnt the case with unsymmetrical alkenes, eg. propene.

the HBr can add across the double bond in two different ways

use Markovnikov’s rule!

Question 19

explain the different types of carbo cations and stability

Answer 19

there are three types: primary, secondary and tertiary

stability: primary < secondary < tertiary

this is because the alkyl groups release electrons onto the positive ion = inductive effect making the carbo cation more stable, this means tertiary are more likely to form

Question 20

expalin the formation of the major product showing the mechanism of HBr and propene

Answer 20

2-bromopropane is formed as the major product as it is formed via the secondary carbo cation which is more stable than the primary carbo cation which the minor product is formed by

Question 21

what are the two ways alcohols can be formed in industry

Answer 21

1) hydrating alkenes in the presence of an acid catalyst such as H₂SO₄
2) steam hydration of ethene

Question 22

explain the conditions needed for and the equation of the steam hydration of ethene

Answer 22

temp = 300 degrees

60 atm pressure

phosphoric acid catalyst

H₂C = C₂H + H₂O ⇔ CH₃CH₂OH

the reaction yield is low (5%) however as reaction is reversible unreacted ethene gas can be recycled improving overall yield to 95%

Question 23

what is addition polymers

Answer 23

lots of alkene monomers (small alkene molecules)

take part in addition polymerisation

Question 24

why are poly(alkenes) unreactive

Answer 24

saturated hydrocarbons

strong carbon to carbon double bonds

non polar so unable to react with nucleophiles

non bio degradable

Question 25

explain poly(ethenes) uses

Answer 25

soft flexible polymer

does have a sharp melting point as it is a mixture of polymer chains with their own melting points

uses : plastic bags and squeezy bottles

Question 26

explain poly(propene) uses

Answer 26

• tough strong polymer

can be recycled by being melted down and remolded

used in ridgid containers and fibres for plastic ropes

Question 27

explain the hydrogenation of ethene

Answer 27

alkene into an alkane

adding of hydrogen

ethene to ethane

conditions:

hydrogen gas, Ni catalyst and 150 degrees

uses: hardening of veg oil to form margarine