Aldehydes and Ketones

Question 1

C=O with neutral nucleophile?

Answer 1

Produces an intermediate containing both a negative and a positive charge empale of a zwitterion, transfer of a proton from Nu to O in two subsequent steps is facilitated by the nucleophile to form a product containing an OH group, all steps reversible, acid catalyst often added to increase rate of addition of neutral nucleophile to C=O bond, net result is addition of NuH across C=O bond

Question 2

C=O with negatively charged nucleophile?

Answer 2

produces an intermediate alkoxide ion, addition of H- or R- is irreversible but additional of CN- is reversible since it is a better leaving group than the other and the C=O bond can be reformed from the intermediate alkoxide ion, in all cases alkoxide ion is protonated in second step

Question 3

Nucleophilic addition reactions of aldehydes and ketones?

Answer 3

Aldehydes react faster than ketones explained by steric and electronic facts in ketones two alkyl or aryl groups hinder approach of nucleophile and rescued the size of partial positive charge on the carbon atom in the C=O bond exerting +I effect making atom less electrophilic

Question 4

Reaction with hydrides?

Answer 4

Aldehydes and ketones react with hydride ions to form alcohol and the C=O bond is reduced

Question 5

Using complex metal hydrides?

Answer 5

Eg NaBH4 or LiAlH4 followed by treatment with acid in a separate stage. Reduction of aldehydes from primary alcohol and the reduction of kenos produces secondary alcohols

Question 6

LiAlH4 vs NaBH4?

Answer 6

LiAlH4 much stronger reducing agents than NaBH4and reacts more rapidly with aldehydes and ketones, Al-H bonds are more reactive and weaker than the B-H bonds in NaBH4 because aluminium is more electropositive than boron, electropositive aluminium ion in [AlH4]- rapidly donates a hydride ion to the C=O bond and so loses a negative charge

Question 7

End of the reaction?

Answer 7

Aqueous acid is added to protonate the intermediate alkoxide ion, the reaction picture is cooled to 0 degrees and the aqueous acid added drowse,

Question 8

Na(CN)BH3?

Answer 8

Another useful hydride less reactive than NaBH4, electron withdrawing CN group stabilises the negative charge on the bon atom and reduces the rate of transfer of a hydride ion, mild reducing agent with molecules containing both an aldehyde and ketone group it can selectively recuse the more reactive aldehyde group

Question 9

Cannizzaro reaction?

Answer 9

hydride ion generated from aldehyde by reaction with hydride ions. Aldehyde must not contains any alpha hydrogen atoms because hydroxide ion mist reacts with the C=O bond of the aldehyde in the first step. Example of a disproportionation reaction

Question 10

Reaction with cyanide to form cyanohydrins?

Answer 10

formation of the cyanohydrin is reversible and the position of the equilibrium depend on size of R groups attached to C=O bond, ketones with large R groups equilibrium favours ketones rather than the cyanohydrin in the ketone the two R groups are separated by a bond angle of 120 degrees but this angle is reduced to 109 in the cyanohydrin and the R groups pushed closer together, if R groups close enough the resulting steric strain causes the equilibrium to favour the ketone, for aldehydes equilibrium favours the cyanohydrin, steric strain formed from an aldehyde is less than that formed from a ketone because a very small hydrogen atom replaces one of the R substituents

Question 11

Reactions with organometallic to form alcohols?

Answer 11

Metals more electropositive than carbon so covalent C-metal bond is polarised with a partial negative charge on the carbon atom, hence why it can reactive with nucleophile or base, Grignard reagents

Question 12

Wittig?

Answer 12

Aldehyde and ketones react with phosphonium ylides to form alkenes in the wittig reaction

Question 13

Mechanism of reaction with metal hydrides?

Answer 13

Involves breaking a B-H or Al-H in the [BH4]- or [AlH4]- ion to donate a hydride ion to the carbon atom of the C=O bond to form a tetrahedral alkoxide ion, protonation of the alkoxide ion forms an alcohol

Question 14

Mechanism of reaction with organometallic to form alcohols?

Answer 14

Organometallic reagents react with aldehydes and ketones to form alcohols, the organic groups acts as a nucleophile and track the carbon atom of the C=O bond to form an alkoxide ion, the alkoxide ion then reacts with H+ in a separate second stage

Question 15

Preparation of phosphonium ylides?

Answer 15

In first step reaction of halogenolkane with a trialkyl or triply phosphine typically triphenylphosphine which is a strong nucleophile produces a phosphonium salt in an SN2 reaction, in the phosphonium salt the positively charged phosphorous atom is the electron withdrawing and this makes the hydrogen atom on the adjacent carbon relatively acidic, treatment with a strong base removes H+ to form the phosphonium ylide

Question 16

Mechanism of Wittig?

Answer 16

Reaction of negatively charged carbon in the ylide with the carbon atom of the C=O form a betaine, this reacts to form an unstable four membered ring called an oxaphosphetane, this break one to form an alkene and triphenylphosphine oxide

Question 17

Addition of water to from hydrides?

Answer 17

Water adds reversibly to C=O of aldehydes and ketones to from hydrates, lone pair of electron on oxygen in water attack carbon atom of C=O to form intermediate consisting positive and negative charge, deprotonation of positively charged oxygen atom using water as the based followed by protonation of the negatively charged oxygen atom using H3O as the acid forms the hydrate

Question 18

Positions of equilibrium?

Answer 18

Detrminin ed by stabilities of carbonyl compound reactive to hydrated steric effects of R groups explain why aldehydes are sually hydrated to greater extent than ketones, more steric strain in a hydrate produces from a ketone so it is less stable relative to the ketone, electronic effects of R groups explains why carbonyls with strongly electron withdrawing groups are usually hydrated

Question 19

Aldehyde or ketone reduction?

Answer 19

Form alcohol with use of NaBH4 or LiAlH4 then H+

Question 20

Alcohol oxidised?

Answer 20

Form ketone or aldehyde with use of CrO3 and H+

Question 21

Aldehyde or ketone with wittig reaction?

Answer 21

Reaction of phosphonium ylide with aldehyde or ketone forms alkene with Ph3P=CHR

Question 22

Aldehyde or ketone with carbon nucelophies?

Answer 22

Form cyanohydrin with HCN, NaCN catalyst

Question 23

Aldehyde or Ketone to form tertiary alcohol?

Answer 23

RLi or RMgBr then H+

Question 24

Reaction or aldehyde or ketone to form acetal?

Answer 24

With 2OH H+ catalysts

Question 25

Reaction or aldehyde or ketone to form hydrate (1,1-diol)?

Answer 25

H2O and H+ catalysts

Question 26

Reaction or aldehyde or ketone to form imine?

Answer 26

RNH2 and H+ catalyst

Reaction with secondary amine forms enamine

Question 27

Reaction or aldehyde or ketone to form thiacetal?

Answer 27

2RSH and H+ catalyst