Deck 8: Halogen derivatives, Hydroxy compounds

Question 1

bp trend for
RX vs corresponding alkane
RX as size of R increases
RX as size of X increases

Answer 1

RX vs corresponding alkane
- RX has higher bp because RX is polar and has pdpd while the alkane is non polar and only has id id

RX as size of R increase
- larger e- cloud, more polarisable stronger id id

RX as size of X increases
- larger e- cloud, more polarisable, id id stronger

Question 2

solubility of RX and why

Answer 2

poor in water
- even though they are polar, they cannot interact with water molecules via hydrogen bonding

soluble in organic solvents
- can interact through id id

Question 3

what do halogenoalkanes react with to undergo elimination

Answer 3

bronsted lowry base

Question 4

just read

Answer 4

if the overall order of reaction is 2, it involves _____ and ______ in the slow single step. hence the reaction is single step and bimolecular
SN2 mechanism

if the overall order of reaction is 1, it involves only the _____ in the slow step. hence the reaction is more than 1 step and unimolecular
SN1 mechanism

Question 5

what kind of enantiomers do SN1 and SN2 mechanisms give

Answer 5

SN1 –> racemic mixture

SN2 –> enantiomerically pure product

Question 6

draw SN2 mechanism and explain

Answer 6

1. C-X bond is polar with C atom having a partial positive charge as it is less electronegative than the halogen
2. nucleophile attacks the partially positively charged C from the side directly opposite the halogen
3. bond breaking and bond making occurs at the same time
4. in the transition state, the Nu gradually loses its negative charge and X gradually gains a full negative charge
5. stereochemical inversion of the tetrahedral arrangement of bonds to the C atom
   INVERSION OF CONFIGURATION

Question 7

for SN2, why must it be backside attack

Answer 7

the partially negatively charged X atom blocks the approach of the nucleophile as they repel each other as they are both electron rich

Question 8

energy profile diagram of SN2

Answer 8

Question 9

what affects reactivity of halogenoalkane in SN2 reaction

Answer 9

1. strength of C-X bond
   - reaction involves heterolytic fission of C-X bond hence when the bond is weaker the reaction is faster
2. steric hindrance of R groups

Question 10

why does C-X bond strength decrease down group 17

Answer 10

1. size of halogen increases and valence orbital becomes more diffuse hence effectiveness of orbital overlap is less effective
2. electronegativity of X decreases down the group and the polarity of the C-X bond falls and ionic character falls so strength falls

Question 11

what does the reactivity of RX for SN2 mechanism not depend on

Answer 11

does not depend on the magnitude of the partial positive charge on the carbon in the C-X bond

Question 12

draw SN1 mechanism and explain

Answer 12

1. C-X undergoes heterolytic fission to give carbocation intermediate
2. carbocation is trigonal planar wrt positively charged C
3. nucleophile attacks the positively charged C of the carbocation from either side of the trigonal plane with equal likelihood
4. yields a racemic mixture where 50% show retention of configuration and the other 50% show inversion of configuration

Question 13

energy profile diagram of SN1 mechanism

Answer 13

Question 14

what affects reactivity of halogenoalkane in SN1 reaction

Answer 14

1. strength of C-X bond (same as SN2)
2. stability of carbocation
   - depends on the rate of formation of the carbocation and a more stable carbocation will be formed faster (eg. 1˚ 2˚ 3˚)

Question 15

how to decide if its SN1 or SN2

Answer 15

SN2
- 1˚halogenoalkane or the one with 3H
- less steric hindrance for backside attack
- carbocation formed is unstable

SN1 or SN2
- 2˚ halogenoalkane

SN1
- 3˚ halogenoalkane
- more stable carbocation formed
- too much steric hindrance for backside attack

Question 16

why some that r supposed to be SN2 become SN1 and just read

Answer 16

can form stable carbocation even as a 1˚carbocation because of the overlap of empty p orbital of the positively charged carbon and the pi electron cloud of the C=C bond or benzene ring

Question 17

just read polyalkylation

Answer 17

SINCE THE 1˚ AMINE FORMED IS ALSO NUCLEOPHILIC

Question 18

what conditions for nucleophiles in nucleo sub vs elimination reactions

Answer 18

OH- act as nucleophile in aq medium for nucleo sub

OH - act as base in alcoholic medium for elimination (eg. remove HX to form C=C)

Question 19

why halogenoarenes (or halogenoalkenes) dont undergo nucleo sub

Answer 19

1. partial double bond character of C-X bond
   - p orbital containing the lp of electrons on the halogen atom overlaps with the pi e cloud of benzene
   - lp of e in the p orbital of the halogen delocalises into the pi electron cloud of the benzene ring
   - partial double bond character in the C-X bond
   - bond is strengthened and not easily broken
2. steric hindrance
   - electronic repulsion due to benzene ring
   - pi e- cloud repel the nucleophile for SN2 mech
   - cannot form stable carbocation for SN1 because the reso stabilisation is destroyed

Question 20

just read

Answer 20

Question 21

bp of alcohols compared to alkanes with similar number of electrons

Answer 21

higher due to ability to form hydrogen bonds

Question 22

factors affecting bp of alcohols

Answer 22

1. length of alkyl chain
   - greater id id
2. branching
   - surface area for intermolecular interactions
3. number of OH groups
   - affects extensiveness of hydrogen bonding

Question 23

solubility of alcohols in water

Answer 23

soluble because can form hydrogen bonds with water

but as the alkyl chain becomes longer, as it is non polar, id id becomes predominant and they get in the way with the hydrogen bonding
too much energy needed to overcome the hydrogen bonds in water and the id id of the molecule so solubility falls

increase in number of OH groups increase the number of hydrogen bonds the alcohol can form with water and affects solubility

Question 24

just read

Answer 24

for alcohols, because they have a polar OH group and a non polar hydrocarbon chain, alcohols can act as solvents to dissolve water soluble and organic substances

Question 25

acidity of phenols, water and alcohol

Answer 25

PWA phenol most acidic alcohol least

Question 26

why NaBH4 and LiAlH4 do not reduce C=C bond

Answer 26

because the C=C is electron rich and NaBH4 and LiAlH4 is also electron rich due to the H- and they repel each other

Question 27

what can H2 Ni and heat, LiAlH4 and NaBH4 reduce and what can it not reduce

Answer 27

can reduce ketone, aldehydes and alkenes but not carboxylic acids NaBH4 only reduce aldehydes and ketones not alkenes and not carboxylic acid LiAlH4 reduce carboxylic and aldehydes and ketones but not alkenes

Question 28

just look

Answer 28

Question 29

phenol solubility in water

Answer 29

only partially soluble as the large non polar benzene ring interferes with the formation of hydrogen bonding between the OH group and water

Question 30

what does the C-O bond on phenols have and why

Answer 30

PARTIAL DOUBLE BOND CHARACTER orbital with the lp of e- on the oxygen atom overlaps with the pi e- cloud of the benzene ring and lp of e- is delocalised into the ring WHICH MEANS PHENOLS DO NOT UNDERGO REACTIONS THAT CLEAVE THE C-O BOND NO NUCLEO SUB NO ELIMINATION

Question 31

Why alcohol react with carboxylic acid to form ester but not phenols

Answer 31

phenol is a weaker nucleophile than an alcohol because the orbital containing the lp of e- on the oxygen atom overlaps with the pi electron cloud of the benzene ring and the lp of e- is delocalised into the ring

Question 32

why does phenol undergo e sub under milder conditions than benzene

Answer 32

hydroxy group is electron donating orbital containing the lp of e- on the oxygen atom overlaps with the pi electron cloud of the benzene ring and the lp of e- is delocalised into the ring, increasing the electron density in the ring making it more susceptible to electrophilic attack

Question 33

why does poly sub occur when Br2(aq) is used but mono sub occurs when Br2 (in CCl4) is used (FOR PHENOLS)

Answer 33

phenol undergoes partial ionisation in water to form the phenoxide ion the aromatic ring in the phenoxide ion is more electron rich and undergoes electrophilic sub more easily than phenols hence forming poly sub products

Question 34

compare acidity of phenols, alcohols and water and explain

Answer 34

Phenol > Water > Alcohol 1. stability of phenoxide ion vs hydroxide ion - phenoxide is more stable due to reso stabilisation - p orbital containing the lp of e- on the oxygen atom overlaps with the pi electron cloud of the benzene ring and the lp of e- is delocalised into the ring - delocalisation of the negative charge on oxygen into the ring and is reso stabilised 2. stability of alkoxide ion (RO-) vs hydroxide ion - RO- has an electron donating alkyl group which increases the electron density and intensifies the negative charge on the oxygen atom and the anion is destabilised