6.1.2 Carbonyl Compounds Flashcards
carbonyls definition
compounds with a C=O
aldehydes or ketones
how to tell if a compound is an aldehyde
if C=O is on the end of the chain with a H attached
name ends in -al
how to tell if a compound is a ketone
if the C=O is in the middle of the chain
name ends in -one
carbonyls solubility in water
smaller are soluble in water
because they can form hydrogen bonds with water
how do pure carbonyls bond
can’t hydrogen bond
bond instead by permanent dipole- dipole forces
how do carbonyls react
C=O is polarised because O is more electronegative than carbon
positive carbon attracts nucleophiles
compare bond strength of C=C and C=O
C=O is stronger than C=C in alkenes so doesn’t undergo addition reactions easily
oxidation of aldehydes
aldehydes -> carboxylic acids
potassium dichromate (VI) solution and dilute sulfuric acid
heat under reflux
orange dichromate reduces to the green Cr3+ ion
how else can aldehydes be oxidised
using Tollen’s reagent
test for aldehydes
Tollen’s reagent
Tollen’s reagent
reagent
condition
reaction
observation
reagent: Tollen’s reagent formed by mixing aqueous ammonia and silver nitrate, active substance complex ion of [Ag(NH3)2]+
conditions: heat gently
reaction: aldehydes only oxidised by tollens into a carboxylic acid, silver (I) ions reduced to silver atoms
observation: with aldehydes silver mirror forms coating the test tube, ketones don’t change
Tollens reagent ionic equation
with ethanal
CH3CHO + 2Ag+ + H2O -> CH3COOH + 2Ag + 2H+
Reduction of carbonyls
reagents
conditions
mechanism
Reagents: NaBH4 in aqueous ethanol Conditions: room temperature and pressure NUCLEOPHILIC ADDITION Aldehydes reduced to primary alcohols Ketones reduced to secondary alcohols Need 2 reducing agents
What are aldehydes reduced to
Primary alcohols
What are ketones reduced to
Secondary alcohols
Reducing agent notation
[H]
When adding make an oxygen sandwich
Examples of reducing agents to reduce carbonyls to alcohols
NaBH4= sodium tetrahydridoborate
LiAlH4 =lithium tetrahydridoaluminate
Describe the nucleophilic addition mechanism for carbonyl reduction
Delta plus delta minus on C=O H- attacks delta plus on carbon Arrow from middle of C=O to the O Lone pair on O draw arrow to H on water molecule Middle of O-H bond draw line to O Forms OH on the carbonyl and OH-
What does NaBH4 contain
Source of nucleophilic hydride ions (H-)
Attracted to positivr carbon in C=O
Hydride ion
H-
Catalytic hydrogenation
Also be reduced using catalytic hydrogenation Reagent: hydrogen and nickel catalyst Conditions: high pressure Examples: CH3CHO + H2 -> CH3CH2OH CH3COCH3 + H2 -> CH3CH(OH)CH3
What is hydroxynitrile
R
|
NC— C —OH
|
HAddition of hydrogen cyanide to carbonyls to form hydroxynitrile
reaction
reagent
conditions
mechanism
Reaction: carbonyl -> hydroxynitrile
Reagent: sodium cyanide (NaCN) and dilute sulfuric acid
Conditions: room temperature and pressure
Mechanism: nucleophilic addition
What do NaCN and H2SO4 provide in hydrogen cyanide to carbonyls mechanism
CN- ions
H+ ions
For second step of mechanism
Naming hydroxy nitriles
CN becomes part of the main chain
Nucleophilic addition mechanism hydrogen cyanide to carbonyls
C=O delta plus on C and delta minus on O
Line from double bond to O
CN- (lone pair also) attacks C+
Lone pair on oxygen donated to H+ from sulfuric acid
Test for carbonyl group
Using 2,4-DNP
Produces orange precipitate
Then if positive use tollens to test if aldehyde or ketone
How can you also identify which carbonyl was used
Add 2,4-DNP then orange ppt will form, derivative of carbonyl group then…
- Filter crystals using Buchner funnel and water pump
- Orange ppt is impure so dissolve crystals in smallest amount of hot solvent (ethanol)
- Put test tube in ice water to allow pure solid to recrystallise
- Filter again using Buchner funnel and water pump, have a pure solid
- Find melting point of pure solid using machine
- Compare to known database of melting points for carbonyl derivatives to identify carbonyls
What is 2,4-DNP
2,4-dinitro phenylhydrazine
2,4-DNP way to remember process
filter dissolve recrystalise filter dry melt check database
