T8: Alkenes

Question 1

alkenes are molecules with…

Answer 1

1 or more C=C double bonds

Question 2

alkenes general formula is C atoms in C=C double bonds have 3 bonds in total which means that the shape around C=C double bonds is what and the bond angle is what

Answer 2

shape = planar
bond angle = 120°

Question 3

C-C single bond: composed of what

Answer 3

σ bond

Question 4

C-C single bond: bond enthalpy / kJ mol-1

Answer 4

+347

Question 5

C=C double bond: composed of what

Answer 5

σ and π bond

Question 6

C=C double bond: bond enthalpy / kJ mol-1

Answer 6

+612

Question 7

what is a σ bond and a π bond

Answer 7

σ bond the same as a regular single covalent bond and it is added to π bond that turns a single bond into a double bond

Question 8

which is stronger, a C-C or C=C bond?

Answer 8

C=C

Question 9

which is stronger, a sigma or pi bond?

Answer 9

σ bond stronger
(612 - 347 = 265)

Question 10

which is more reactive, a sigma or pi bond?

Answer 10

π more reactive
(needs less energy to break)

Question 11

which is more reactive an alkane or an alkene?

Answer 11

alkenes are more reactive
(thermal cracking is used to turn alkanes into alkenes)

Question 12

orbital definition

Answer 12

a region of space where up to 2 electrons can be formed

Question 13

covalent bonds are made from

Answer 13

shared pairs of electrons

Question 14

a C=C double bond is made from

Answer 14

2 shared pairs of electrons

Question 15

how is the π bond formed

Answer 15

P orbital overlaps sideways on to form a π bond

Question 16

why is there no rotation around a C=C double bond

Answer 16

the π bond is formed by overlap of p orbitals
if there was any rotation, the p orbitals would no longer overlap and the π bond would break

Question 17

C=C are areas with…

Answer 17

high electron density (e- rich -> 4e- between 2 C atoms)

Question 18

electrophile definition

Answer 18

an electron pair acceptor

Question 19

3 types of electrophiles

Answer 19

• hydrogen halides (HCl, HBr)
• halogens (Cl2, Br2)
• sulphuric acid (H2SO4)

Question 20

electrophilic addition with hydrogen halides: reagent

Answer 20

HCl, HBr

Question 21

electrophilic addition with hydrogen halides: conditions

Answer 21

none

Question 22

electrophilic addition with hydrogen halides: type of product

Answer 22

haloalkane

Question 23

electrophilic addition with hydrogen halides: the electrophile in this reaction is…

Answer 23

HBr

Question 24

electrophilic addition with hydrogen halides: this reaction is an addition reaction because…

Answer 24

a H atom and a Br atom have been added

Question 25

electrophilic addition with hydrogen halides: overall reaction

Answer 25

CH2=CH2 + HBr -> CH3CH2Br

Question 26

what is formed in the middle step of electrophilic addition with hydrogen halides

Answer 26

an intermediate carbocation

Question 27

what are alkyl groups

Answer 27

Methyl (CH3), Ethyl (CH2CH3), Propyl (CH2CH2CH3)

Question 28

what is a positive inductive effect

Answer 28

the ability to release electron density through a covalent bond

Question 29

stability: carbocation rank

Answer 29

primary carbocation (least stable) secondary carbocation tertiary carbocation (most stable)

Question 30

why are tertiary carbocations more stable than primary carbocations

Answer 30

there is a positive inductive effect from 3 alkyl groups in 3° carbocation compared to 2 alkyl groups in 2° carbocations

Question 31

explain why 2-bromopropane is the major product in this reaction (outline the mechanism to form both possible products when propane reacts with HBr)

Answer 31

2-bromopropane is made from a secondary carbocation 1-bromopropane is made from a primary carbocation secondary carbocations are more stable than primary carbocations due to the positive inductive effect of the 2 alkyl groups compared to 1

Question 32

electrophilic addition with halogens: reagent

Answer 32

Cl2, Br2

Question 33

electrophilic addition with halogens: conditions

Answer 33

none

Question 34

electrophilic addition with halogens: type of product

Answer 34

haloalkane

Question 35

electrophilic addition with halogens: bromine reacts with alkenes even though bromine is a non-polar molecule. explain why bromine reacts with alkenes

Answer 35

the C=C bond is e- rich and induces a dipole in the Br2 partially positive Br attracted to C=C double bond

Question 36

electrophilic addition with halogens: overall reaction

Answer 36

CH2=CH2 + Br2 -> CH2BrCH2Br

Question 37

reagent used to test for alkenes

Answer 37

Br2

Question 38

observation when alkene present

Answer 38

orange to colourless

Question 39

observation when alkene not present

Answer 39

no visible change

Question 40

structural formula for H2SO4

Answer 40

H | OSO3H

Question 41

electrophilic addition with sulphuric acid: reagent

Answer 41

H2SO4

Question 42

electrophilic addition with sulphuric acid: conditions

Answer 42

none

Question 43

electrophilic addition with sulphuric acid: type of product

Answer 43

alkyl hydrogen sulphate

Question 44

electrophilic addition with sulphuric acid: overall reaction

Answer 44

CH2=CH2 + H2SO4 -> CH3CH2OSO3H

Question 45

this is 1 way to make alcohols from alkenes in a 2-step process: step 1

Answer 45

alkene + H2SO4 -> alkyl hydrogen sulphate

Question 46

this is 1 way to make alcohols from alkenes in a 2-step process: step 2

Answer 46

alkyl hydrogen sulphate + H2O -> alcohol + H2SO4 (hydrolysis)

Question 47

role of the H2SO4 in the 2-step transition from alkene to alcohol

Answer 47

H2SO4 is reproduced so it is called a catalyst

Question 48

direct hydration: reagent

Answer 48

H2O(g)

Question 49

direct hydration: conditions

Answer 49

conc H3PO4 catalyst

Question 50

direct hydration: type of product

Answer 50

alcohol