topic 17b: carbonyls Flashcards
aldehyde
r-c=o-h
ketone
r-c=o-r’
naming aldehyde
- no need to include number
- prefix-anal
naming ketone
-prefix-an-number-one
carbonyls IMFs
- LFs
- permanent dipole due to delta positive C, delta negative O
compare a carbonyl to an alcohol with similar MR?
- same LF
- alcohol has HYDROGEN BONDING
CARBONYLS H bonding with water
- delta negative O, lone pairs H bond with delta positive H on water
small carbonyls solubility
soluble in water due to H bonds; delta h in water attracted to lone pair on oxygen atom of carbonyl
LARGER carbonyls solubility
- less soluble
- longer hydrocarbon chains which are hydrophobic and NON POLAR(can’t h bond w water)
- proportion of molecule that can H bond w water smaller
- strength of H bond with water weaker than IMFs between each molecule
- so energetically unfavourable
aldehydes + acidified potassium dichromate (VI)
- OXIDATION
- to form carboxylic acid
- orange to green (cr2o7 2- to cr3+)
ketone + acidified potassium dichromate (vi)
NO CHANGE, stays orange
- ketons are resistant to oxidation
- no readily available h atom
tollens reagent is …
ammoniacal silver nitrate
tollens + aldehyde
- heat
- oxidation of aldehyde
- silver mirror formed
what is produced with tollens
- reduction of Ag+ to Ag
- aledhyde oxidised to carboxylic acid
what is fehlings
- cu2+ in sodium hydroxide
what is benedicts
cu2+ in sodium carbonate
aldehyde + fehling/benedicts
HEAT
- reduction. of cu2+ to cu+
-blue to brick red ppt
- aldehyde oxidised to carboxylic acidf
aldehydes reduce to…
primary alcohol
ketones reduce to
secondary alcohol
reduction of carbonyl
- LiAlH4 in dry ether
- EQN: + 2[H]
- nucleophilic addition
carbonyl + HCN in the presence of KCN
- nucleophilic addition
- c(OH)(CN)(R2)
carbonyl + 2,4 DNP
- orange ppt
USEFUL AS COXYLIC ACIDS AND ESTER DONT GIVE POSITIVE
Iodoform test
- iodine + carbonyl + add sodium hydroxide until iodine decolourises
- pale yellow PPT formed
- ‘medical’ Smell
iodoform test product
CHI3
hazard of KCN
toxic
why presence of KCN
HCN alone too weak; KCN dissociates more to produce CN- nucleophile
if the aldehyde is not the principle group it is?
oxo
what is formed in the HCN addition
hydroxy alkane-nitrile
effect of pH on the HCN nucleophilic addition reaction
CONSIDER EQM:
HCN -> H+ + CN-
NEED HIGH PH
- low conc h+
- shift position of equilibrium to the right to get a high conc of CN- nucleophile to attack c +
NEED LOW PH
- need acid so that the anion intermediate can be PROTONATED, if not might recert to carbonyl
USE PH 8 ish
HCN mechanism (draw)
- lone pair on CN attacks delta + carbon of c=o
- c=o breaks
- INTERMEDIATE: C-O:-, C—N
- lone pair on o goes to h. h-cn breaks, goes to cn
- final is sa hydroxy nitrile
chirality eith hcn addition
- trigonal plana carbonyl group
- attack by CN- nucleophile can happen above or below plane w equal probability
- racemic mixture produced
why LiAlH4 in dry ether
water would destroy reagent
what must you do to generate final compound for LiAlH4
-add acid or water
what feature of carbonyl make snucleophilic attack possible
c=o
- o more elctroneagtive so delta - and you have delta + c
- so c+ vulnerable to attack from nucleophile
why can c=c not undergo nucleophilic addition
- non polar bond
- rich electron density so nucleophile not attracted
why dont ketones readily oxidise
no H to lose
redox equation eg methanal and dichromate ion (ACIDIC)
ORANGE TO GREEN
3CH3CHO + Cr2O72- + 8H+ -> 3CH3COOH +2Cr3+ + 4H2O
other reducing agent
NaBH4
Why do NaBH4 and LiAlH4 not reduce C=C but do reduce C=O
- C=C non polar so nucleophile not attracted
- o more eneg than c so h- HYDRIDE nucleophile attracted to delta + c
iodoform only works for
methyl ketones
- ETHANAL
- secpndary alcohol
iodoform reaction equation
carbonyl + I2 + alkali = CHI3 + carboxylate salt + ionic salt + water
iodoform only tests for…
methyl ketones
give the steps for how 2,4 DNP can distinguish between aldehyde and ketone (4)
- add DNP to solutions
- forms a yellow orange preceipitate
- filter and recrystallise
-determine melting temperature - compare to database
why is there a reacemic mixture formed in the HCN mechanisms? (2)
- planar around carbonyl carbon
- equal likelihood of nucleophile attack above and below plane
which is stronger out of c=o c=c
- c=o
- doesnt readily undergo addition
why are carbonyls vulnerable to attack by nucleophiles
- o more eneg than c
- dipoles delta + c, delta - o
- nucleophiles attracted to delta + c
what do ketones do (oxidation)
RESIST OXIDATION
aldehyde oxidation
- to carboxylic acid
- acidified potassium dichromate
- heat under reflux
- orange Cr2O7 2- to Cr 3+ GREEN
what happnens in tollens
- test for aldehydes
- heat gently
- oxidises aldehyde to carboxylic acid
- ag+ reduced to Ag so silver mirror formed
what happens in fehlings
- cu2+ blue reduced to RED PPT Cu2O
- aldehyde oxidised to carboxylic acid
aldehydes reduce to …
primary alcohols
ketones reduce to …
secondary alcohols
what is the product formed when u nucleophilically add CN
- hydroxy nitrile
reagent for cn mechanism
- HCN in the presence of KCN
- KCN increases conc of CN- nucleophile needed for step 1
lone pair on cn- is on…
c
whcih is the only aldehyde that does iodoform
ETHANAL
- there has to be a methyl group next to the c=o
2,4 DNP tests for …
any carbonyl
nucleophile defintion
- has a lone pair
- attracted to and ATTACKS regions of +
