Alkenes

Question 1

why is a cis alkene less stable than a trans alkene

Answer 1

experiences steric strain
2 alkyl groups on the same side of the C atom in the cis alkene, their electron clouds overlap each other and experience repulsion

Question 2

why are alkenes highly reactive

Answer 2

has presence of electron rich C=C bond that attracts electrophiles

Question 3

what are electrophiles

Answer 3

electron pair acceptors that are attracted to electron rich sites
are electron deficient

Question 4

what makes a molecule electron deficient

Answer 4

has a positive charge
partial positive charge
incomplete octet

Question 5

how are alkenes nucleophiles

Answer 5

able to donate electron pairs to electron deficient atom to form a covalent bond

Question 6

how is a molecule electron rich

Answer 6

has negative charge, partially negative charge, has lone elctron pair

Question 7

what are addition reactions

Answer 7

pi bond in the C=C bond is broken, forming 2 sigma bonds and forming 2 C-C bonds
there is a decrease in saturation
original sigma bond in unsaturated C=C bond remains unaffected

Question 8

what is heterolytic fission

Answer 8

bond is broken in a way that both of the 2 bonding electrons go to one atom

Question 9

why is the first step in electrophilic addition a slow step

Answer 9

the step requires higher activation energy due to the heterolytic fission

Question 10

what are unsymmetrical alkenes

Answer 10

one that does not display cis trans isomerism
has more than 2 types of groups attached to the C=C
has 2 of the same group attached to the same C atom

Question 11

when is a carbocation stable

Answer 11

the greater the number of electron donating alkl groups attached to the positively charged C atom, the greater the dispersal of the positive charge, the more stable the carbocation

Question 12

how can stereochemistry allow a racemic mixture from an addition product

Answer 12

alkene’s cis and trans alkene react with electrophile to give same carbocation
nucleophile can attack the positively charged C of the trigonal planar carbocation from either side of trigonal plane with equal likelihood
reaction from each side both form enantiomers
since nucleophilic attack on both sides happen with equal likelihood will result in 50:50 ratio of enantiomeres

Question 13

why are inert solvents used for halogen additions

Answer 13

allows both halogen and the alkene to be dissolved

Question 14

why is fluorine not used in halogenation

Answer 14

it reacts explosively with alkenes

Question 15

why is iodine not used in halogenation

Answer 15

reaction is thermodynamically unfavourable
vicinal diodides are unstable and will decompose back to alkene and iodine

Question 16

how do partial charges arise on Cl2/Br2 during electrophilic addition of halogens in CCl4

Answer 16

pi electron cloud of alkene repels electron cloud of Br2/Cl2, distorting and polarising Br2/Cl2 electrons cloud, allowing partial charges to arise on each atom of Br2/Cl2

Question 17

relation between reduction and number of C=C bonds

Answer 17

one mole of C=C bond requires one mole of H2 to reduce so amount of H2 reacted per mole of alkene is also the number of C=C bonds present

Question 18

relation between halogenation and number of C=C bonds

Answer 18

1 mol of Br2 reacts with 1 mole of C=C, so amount of Br2 reacted with one mole of C=C, is equal to number of C=C in the alkene

Question 19

distinguising between alkenes and alkanes using halogenation

Answer 19

test: add br2 in ccl4 drop wise to each sample in a test tube at room temp
obeservations:
alkane: no rapid decolourisation of orange red br2
alkene: rapid decolorisation of orange red br2

Question 20

distinguishing between alkanes and alkenes using hot acidic KmNO4

Answer 20

test: add 2 drops of Kmno4 acidified with dilute h2so4 to each sample in a test tube and heat each reaction mixture in a hot water bath
observations
alkenes: decolourisation of purple kmno4 and brown black ppt of mno2 observed
alkanes: no decolourisation of purple kmno4 and no brown black ppt of mno2 observed

Question 21

distinguishing alkanes and alkenes but in gaseous state

Answer 21

test: bubble a sample of each gase through br2 (aq) in separate test tubes at room tempalkane: alkane: no rapid decolourisation of orange red br2
alkene: rapid decolorisation of orange red br2

Question 22

distinguishing between a normal alkene and a terminal alkene

Answer 22

test: add 2 drops of KMno4 acidified with dilute h2so4 to each sample in a test tube and heat each reaction mixture in a hot water bath
bubble any gas produced through limewater
observations:
terminal alkene: decolourisation of purple kmno4 and brown black ppt of mno2 observed
white ppt observed in limewater
normal alkene: decolourisation of purple kmno4 and brown black ppt of mno2 observed
no white ppt observed in limewater