3.3.4 Alkenes Knowledge

Question 1

what are alkenes?

Answer 1

hydrocarbons with one or more C=C double bonds

Question 2

what is the shape around C atoms in a C=C bond and why?

Answer 2

C atoms in C=C double bonds have 3 bonds in total, so the shape around the double bond is planar and the bond angle is 120

Question 3

what are the 2 bonds that make up a C=C bond?

Answer 3

sigma and pi

Question 4

what is a single covalent bond made from, in terms of orbitals?

Answer 4

The single bond is made from a sigma overlap; an s orbital from each carbon overlap and their electrons share the region of space between the orbitals

Question 5

what is a double covalent bond made from, in terms of orbitals?

Answer 5

The double bond is made from sigma and pi overlap; a pi overlap is when p orbitals overlap above and below the carbons

Question 6

are single or double covalent bonds stronger?

Answer 6

double

Question 7

are sigma or pi bonds stronger?

Answer 7

sigma

Question 8

are sigma or pi bonds stronger, and which is more reactive?

Answer 8

A sigma bond is stronger than a pi bond, which means pi bonds are more reactive (easier to break)

Question 9

why are alkenes more reactive than alkanes?

Answer 9

• Double bonds are stronger than single
• A sigma bond is stronger than a pi bond, which means pi bonds are more reactive (easier to break)
• alkanes have only single bonds

Question 10

what reagent is used in the chemical test for alkenes?

Answer 10

bromine

Question 11

what are the observations for the chemical test for alkenes?

Answer 11

Bromine is an orange liquid; the haloalkanes formed when bromine reacts with an alkene are colourless.

Question 12

write a balanced equation for the reaction that occurs in a chemical test for alkenes, of ethene

Answer 12

H2C=CH2 + Br2 ⇒ CH2BrCH2Br

Question 13

what is the reagent required for the electrophilic addition of hydrogen halides and alkenes?

Answer 13

HCl or HBr

Question 14

what are the conditions required for the electrophilic addition of hydrogen halides and alkenes?

Answer 14

none

Question 15

what is the product of the electrophilic addition of hydrogen halides and alkenes?

Answer 15

haloalkane

Question 16

what is the electrophile in the electrophilic addition of hydrogen halides and alkenes?

Answer 16

HBr or HCl, depending on which is sued

Question 17

what is the reagent required for the electrophilic addition of halogens and alkenes?

Answer 17

Cl2 or Br2

Question 18

what are the conditions required for the electrophilic addition of halogens and alkenes?

Answer 18

none

Question 19

what is the product of the electrophilic addition of halogens and alkenes?

Answer 19

haloalkane

Question 20

why does bromine react with an alkene, even though bromine is a non-polar molecule?

Answer 20

The C=C bond is electron rich and induces a dipole in Br2; the delta positive Br is attracted to the C=C double bond.

Question 21

what is the reagent required for the electrophilic addition to create an alkyl hydrogensulfate?

Answer 21

H2SO4

Question 22

what are the conditions required for the electrophilic addition of alkenes and H2SO4?

Answer 22

none

Question 23

what is the product of the electrophilic addition of alkenes and H2SO4?

Answer 23

alkyl hydrogensulfate

Question 24

draw the displayed formula for H2SO4

Answer 24

Question 25

draw the mechanism for the reaction between H2SO4 and propene

Answer 25

electrophilic addition

Question 26

draw the mechanism for the reaction between bromine and ethene

Answer 26

electrophilic addition

Question 27

draw the mechanism for the reaction between HCl and ethene

Answer 27

electrophilic addition

Question 28

what is a carbocation?

Answer 28

a positive molecular ion (cation) where the charged atom is carbon

Question 29

what is the positive inductive effect?

Answer 29

the ability to lose electron density through a covalent bond

Question 30

what is the reactive intermediate in an electrophilic addition reaction of hydrogen halides?

Answer 30

carbocation

Question 31

what are the qualities of reactive intermediates?

Answer 31

Reactive intermediates are unstable and quickly either react further or decompose into their reactants

Question 32

what does the stability of a carbocation depend on?

Answer 32

Carbocations have different stabilities depending on the number of alkyl groups bonded to the positive C atom

Question 33

why does the number of alkyl groups bonded to the C+ atom affect a carbocation’s stability?

Answer 33

Alkyl groups push electron density onto whatever they are bonded to; positive inductive effect

Question 34

why are tertiary carbocations more stable than primary?

Answer 34

because there is a greater positive inductive effect from 3 alkyl groups in tertiary carbocation, than 1 alkyl group in primary carbocation

Question 35

will the major or minor product of electrophilic addition form in greater quantity?

Answer 35

major

Question 36

in the electrophilic addition of hydrogen bromide and prop-1-ene, what will be the major and minor product and why?

Answer 36

2-bromopropane is formed from a secondary carbocation, where 1-bromopropane is formed from a primary carbocation. The secondary carbocation is more stable than the primary due to the positive inductive effect. Therefore, 2-bromopropane is the major product.

Question 37

what is the mechanism to make an alcohol from an alkyl hydrogen sulfate?

Answer 37

nulcophilic substitution/hydrolysis

Question 38

what are the 2 steps to make an alcohol from an alkyl hydrogen sulfate?

Answer 38

- Step 1: alkene + H2SO4 ⇒ alkyl hydrogensulfate - Step 2: alkyl hydrogensulfate + H2O ⇒ alcohol + H2SO4

Question 39

what is the role of the sulfuric acid in the nucleophilic substitution of an alkyl hydrogen sulfate to make an alcohol?

Answer 39

catalyst

Question 40

out line a mechanism to make ethanol from ethene using H2SO4

Answer 40

Question 41

what is the reagent required for the direct hydration of an alkene?

Answer 41

h2o(g) - steam

Question 42

what conditions are required for the direct hydration of an alkene?

Answer 42

concentrated H3PO4 catalyst

Question 43

what is the product of the direct hydration of an alkene?

Answer 43

alcohol

Question 44

outline a mechanism for the direct hydration of ethene

Answer 44

Question 45

why is the direct hydration of alkenes to alcohols preferred industrially?

Answer 45

- there are no waste products and so has a high atom economy - separation of products is easier and cheaper

Question 46

why are poly(alkenes) unreactive?

Answer 46

due to the strong C-C and C-H bonds

Question 47

what is poly(chloroethene)'s properties?

Answer 47

- waterproof - electrical insulator - doesn't react with acids

Question 48

what is poly(chloroethene) like in its pure form?

Answer 48

it is a rigid plastic

Question 49

why is poly(chloroethene) a rigid plastic in its pure form?

Answer 49

the strong IMF between polymer chains prevents them moving over each other

Question 50

what is pure poly(chloroethene) used for?

Answer 50

to make uPVC window frame coverings and guttering

Question 51

what happens if a plasticiser is added to a polymer?

Answer 51

the IMF are weakened which allows the chains to move more easily, resulting in more flexibility in the polymer

Question 52

what is plasticised poly(chloroethene) used for?

Answer 52

insluation on electical wires and waterproof clothing