Alkenes

Question 1

General formula of alkenes

Answer 1

CnH2n

Question 2

Functional group

Answer 2

C=C

Question 3

What types of isomerism can alkenes exhibit

Answer 3

Both constitutional and cis-trans

Constitutional:
- chain isomerism, different structure of carbon skeleton
- positional isomerism, different position of the double bond in the same carbon chain

Cis trans:
- restricted rotation about C=C double bond due to pi bonds
- each C bonded to 2 different groups

Question 4

What are the physical properties of alkenes?

Answer 4

Similar to alkanes, but slightly lower boiling and melting points than corresponding alkanes

Question 5

Which isomer has a larger dipole moment

Answer 5

Cis isomer. If the groups on the trans isomer are the same, then it has no dipole moment

Question 6

How does the reactivity of alkenes compare to alkanes and why?

Answer 6

Much more reactive than alkanes,
Due to
1.) high electron density about the carbon-carbon double bond, so they act as Lewis bases(nucleophiles) and are susceptible to reactions with electrophiles

Question 7

What reaction do alkenes undergo

Answer 7

Electrophilic addition at C=C bond
- involves the breaking of the weaker pi bond to form two strong sigma bonds

Question 8

What are the reagents and conditions for electrophilic addition of bromine

Answer 8

1.) Br2(l), rtp or Br2 dissolved in CCl4, rtp
2.) Br2(aq), rtp
3.) HBr(g) or HBr dissolved in CCl4, rtp

Question 9

What are the chemical tests for alkenes involving bromine and what are the observations

Answer 9

1.) Br2(l) used: decolourisation of reddish brown bromine
2.) Br2 dissolved in CCl4: decolourisation of orange red bromine
3.)Br2(aq): decolourisation of orange bromine