Composés carbonyles Flashcards
Give general formula for saturated aliphatic carbonyl compounds
Aldehyde: CnH2n+1CHO
Ketone: CnH2n+2CO
What is one big difference between aldehyde and ketone?
- Aldehyde hv carbonyl group at end of chain
- Ketone hv carbonyl group in non-terminal posit n in chain
*NOTE: carbonyl group refers to C=O, not the same as carbonyl compounds!
Describe nomenclature of aldehydes and ketones
aldehyde:
- -CHO carbon is numbered carbon 1
- name ends in -al (eg propanal)
ketone:
- parent chain is longest one containing ketone group
- numbering begins at end nearer carbonyl carbon (if needed, number indicates pos n of carbonyl group in chain)
- name ends in -one (eg butanone)
What is the difference between aromatic and aliphatic ketone/aldehyde
- aromatic: -CHO/-C=O grp directly attached to a benzene ring
- aliphatic: -CHO/-C=O grp NOT directly attached to a benzene ring (attached to alkyl grp)
NOTE: if alkyl grp also hv benzene ring somewhere, compound considered aromatic
Explain boiling points of carbonyl compounds
- polar molecules bcos large diff in electro-vity btw C & O, so C=O bond is polar
- (more than alkane of comparable Mr) More energy needed overcome stronger pd-pd attract n btw aldehyde/ketone molecules vs weaker id-id attract n btw non-polar alkane molecules
- (less than alcohol of similar Mr) More energy needed overcome stronger H bonds btw alcohol molecules vs weaker pd-pd attract n btw aldehyde/ketone molecules
NOTE: aldehyde and ketone cnt form H bonds w itself bcos H not covalently bonded to O, so cnt form H bonds
Explain solubility of carbonyl compounds in water
- Lower aldehyde, ketones (eg methanal, ethanal, propanone) r completely miscible in water (no separate layers
bcos
- energy released by form n H bonds btw aldehyde/ketone & water molecules enough overcome intermolecular H bond btw water molecules & pd-pd attract n btw aldehyde/ketone molecules
NOTE: aldehyde/ketone cnt form H bond w itself but can form H bond w water molecules
- Solubility of aldehydes/ketones in water decrease for higher aldehyde/ketone members (more C)
bcos
- larger non-polar R grp make molecule more hydrophobic
- main interact n btw larger aldehyde/ketone molcules bcome id-id instead
- energy released fr id-id attract n btw aldehyde/ketone & water molecules insufficient overcome intermolecular H bond btw water molecules & pd-pd attract n (& id-id attract n ) btw aldehyde/ketone molecules
Carbonyl compounds are formed by …
oxidat n of alcohols
Name reagent, conditions, observation and type of reaction of preparing aldehydes from primary alcohols
R&C: K2Cr2O7(aq), H2SO4(aq), heat WITH immediate distillat n
Observ n: orange K2Cr2O7 sol n turn green
Type: oxidat n
Name reagent, conditions, observation and type of reaction of preparing ketones from secondary alcohols
R&C: K2Cr2O7(aq)/KMnO4(aq), H2SO4(aq), heat
Observ n: Orange K2Cr2O7 sol n turn green/ Decolour n of purple KMnO4 sol n
Type: oxidat n
What reaction do carbonyl compounds undergo in general?
Nu Add n
Explain how susceptible to nucleophilic attack are carbonyl compounds and alkenes
Carbonyl cpd
- e-deficient, bcos bonded to electro-ve O atom
- more susceptible to nucleophilic attack
Alkene
- C=C is e-rich
- not susceptible to nucleophilic attack
Name reagent, condition, type of reaction and application of carbonyl cpd + HCN
R&C: HCN w trace amt NaCN (or NaOH) as catalyst, cold
type: Nu add n
Applicat n: increase no of C atom (resulting nitrile can b hydrolysed form RCOOH or reduced to amine)
Describe a general nucleophilic addition mechanism
Step 1: Nu, CN-, attack e- deficient carbonyl C atom, form tetrahedral anion intermediate (C=O bond w dipoles, full arrow fr bond to O) (SLOW)
Step 2: protonat n of intermediate to form stable add n pdt (lp e- fr O attack H fr HCN, full arrow fr H-C bond to CN)
Why carbon atom attacks carbonyl compound instead of nitrogen in CN-?
bcos
- C less electro-ve
- lp on C can b donated more readily form dative bond to carbonyl grp
- C is btr Nu
What is the role of NaOH/NaCN as reagent in nucleophilic addition?
- NaOH
- trace NaOH neutralise H+ (fr HCN), decrease [H+]
- By LCP, eqm pos n shift to increase [H+], so increase in [CN-] & rate of rxn
- said to b base-catalysed - NaCN
- soluble, fully dissociates produce initial CN- for Nu attack on e- deficient carbonyl C
- homogenous catalyst (CN- is regenerated in second step)
What mixture is formed when chiral C is obtained from nucleophilic addition? Why?
racemic mix
bcos
- equal probability CN- Nu attack trig planar sp2 hybridised e- deficient carbonyl C atom fr top & bottom of plane
=> form equal amt (+) & (-) enantiomers, thus racemic mix
Explain relative reactivity between aldehydes and ketones
aldehyde generally more reactive than ketone in Nu add n bcos
- steric effect
- 2R grp in ketone cause greater steric hindrance vs aldehydes w oni 1 grp
- more difficult for Nu to attack e- deficient carbonyl C in Nu add n rxn - Electronic effect
- ketone hv 2 e- donating alkyl/R grp, makes carbonyl C less e- deficient/e-philic
=> less susceptible to Nu attack vs aldehyde w oni 1 grp
Name reagent, condition and type of reaction of hydroxynitrile to hydroxyamine
R&C:
- LiAlH4 in dry ether
OR
- H2 w Pd/Pt catalyst
OR
- H2 w Ni catalyst, heat
type: reduct n
Name reagent, condition and type of reaction of hydroxynitrile to RCOOH
R&C: HCl (aq)/H2SO4 (aq), heat under reflux
type: acidic hydrolysis
Name reagent, condition and type of reaction of hydroxynitrile to RCOO-
R&C: NaOH (aq), KOH(aq), heat under reflux
type: basic hydrolysis
Describe oxidation reaction of carbonyl compounds
only aldehyde undergo oxidation give RCOOH
ketones DON’T
Name reagent, condition, observation and type of reaction of aldehyde to RCOOH
R&C:
- K2Cr2O7(aq), H+, heat
OR
- KMnO4 (aq), H+, heat
Observ n:
orange K2Cr2O7 sol n turn green
OR
decolourisat n purple KMnO4 sol n
type: oxidat n
Describe reduction reaction of carbonyl compounds
- aldehydes reduce to form primary alcohol
- ketone reduce to form secondary alcohol
Name reagent, condition and type of reaction of carbonyl compound to alcohol
R&C:
LiAlH4 in dry ether
OR
NaBH4 in ethanol
OR
H2 w Pd/Pt catalyst
OR
H2 w Ni catalyst, heat
type: reduct n
NOTE: NaBH4 can oni reduce carbonyl compounds! (not RCOOH, alkenes, etc.)
Why is LiAlH4 a stronger reducing agent than NaBH4?
- bcos weaker Al-H bond vs B-H bobnd, so easier to break produce H- nucleophile (due to extent of orbital overlap)
- bcos low electro-vity of Al vs B -> shift e- density to H more, make it btr donor of e-
Carbonyl compounds can be reduced to alcohols using LiAlH4. But, alkenes cannot be reduced by LiAlH4. Why?
- alkene no hv e- deficient C atom attracting Nu
- alkene contain C=C w e- rich π e- cloud, repels any Nu fr LiAlH4
- so, alkene not susceptible to Nu attack by H- fr LiAlH4
Define condensation reaction
rxn in which 2 molecules added, accompanied by eliminat n of small molecules eg H2O, NH3, etc.
Name applicat n, test, observ n & type of rxn of carbonyl cpd + 2,4-DNPH
applicat n: chemical test for BOTH aldehydes & ketones
test: add 2,4-DNPH, heat
observ n: orange crystals of 2,4-dinitrophenylhydrazone formed
type: condensat n
Name applicat n, test, observ n & type of rxn of carbonyl cpd + tollens’ reagent
applicat n: chemical test for aromatic & aliphatic aldehydes (distinguish fr ketones)
test: add Tollens’ reagent (aq NH3 w AgNO3), heat
observ n: for aldehyde, silver mirror/black ppt formed
type: mild oxidat n
NOTE: mol ratio RCHO(aq) = 2Ag(s)
Name applicat n, test, observ n & type of rxn of carbonyl cpd + Fehling’s reagent
applicat n: chemical test for aliphatic aldehyde ONLY (distinguish fr aromatic aldehyde)
test: add Fehling’s reagent, heat
observ n: brick red ppt Cu2O formed in presence of aliphatic aldehyde
type: mild oxidat n
NOTE: mol ratio RCHO(aq) = Cu2O (s)
Name applicat n, test, observ n & type of rxn of carbonyl cpd + I2
(iodoform test)
applicat n: identifies aldehydes, ketones w structure CH3 C=O R
where R = H, alkyl, aryl (for aldehydes, only ethanal possible) AND alcohols w structure CH3 CH(OH) R
test: add I2 in NaOH (aq), heat
observ n: pale yellow ppt of CHI3 observed
type: oxidat n
NOTE:
1. carboxylate salt pdt hv 1 less C atom than reactant
2. carbonyl cpd w CH2ICO-, CHI2CO-, CI3CO- grp oso register +ve test
3. iodoform test CANNOT test for carbox acid, acyl chloride, esters
4. mol ratio R C=O CH3 = CHI3
