alkenes

Question 1

Describe hybridisation, shape and bonding of alkene

Answer 1

1. C involved in C=C bond are sp2 hybridised
2. sigma bond is formed w/ e/o by head-on overlap of sp2 orbitals
3. pi bond is also formed by side-on overlap of unhybridised p orbitals orientated perpendicular to the sp2 plane.

Question 2

Why is there cis-trans isomerism?

Answer 2

1. restricted rotation about C=C bond
2. 2 diff group attached to each C in C=C

Question 3

Why does MP + BP increase as number of C atoms increase within homogenous series?

Answer 3

1. no of e- increases
2. size of e- cloud increases
3. stronger dispersion forces btw mlc

Question 4

why does trans isomer have higher MP?

Answer 4

1. pack better than cis isomer due to higher symmetry of trans isomer
2. better packing in solid state means dispersion forces work more effectively

Question 5

why alkenes undergo E.A?

Answer 5

e- rich C=C attracts E+

Question 6

Which is a more stable carbocation/ favoured product?

Answer 6

1. carbocation w/ more e- donating alkyl grps disperse +ve charge on central C atom to a greater extend
2. increased stability

*markovnikov’s rule (addition of HX to alkene): H atom adds to C atom of double bond that holds the greater no. of H atoms

Question 7

why does E.A forms racemic mixture?

Answer 7

1. geometry about positively charged C is planar
2. equal chances of being attacked from the top or bottom of Nu-
3. 2 enantiomers are produced in equimolar ratio

Question 8

LiAlH4 reacts with carbox but not with alkenes. Explain why

Answer 8

LiAlH4 is a source of electron rich hydride ions which reacts with electron deficient
C in the COOH.

The pi bonds in alkenes are electron rich and the C=C have no electronegativity
difference, so alkenes cannot react with nucleophilic hydride ions.

Question 9

Why does LiAlH4 react with benzaldehyde but not alkenes?

Answer 9

(*LiAlH4 and NaBH4 are R.A)

1. H- providers that attacks partial charge +ve species (symbol) species for reduction
   2.