Topic 17 + grignard reagents Flashcards
Why are carboxylic acids acidic?
because they can donate the H in the -COOH group
What happens to the H in the -COOH when carboxylic acids form salts?
the H is replaced by a positive metal ion
—-O(-)Na(+) [sodium ethanoate eg.)
Boiling point of carboxylic acids
higher boiling points that alcohols (and alkanes)
- form hydrogen bonds between two molecules of carboxylic acid to produce a dimer
(between delta positive hydrogen and lone pair on oxygen) AKA between the acidic H (from C-OH) and carbonyl O
- also additional london forces?
Solubility of carboxylic acids in water
hydrogen bonds form between carboxylic acid and water molecules
solubility decreases as chain length increases
because the alkyl groups interrupt the hydrogen bonding for water molecules but then for london forces in replacement. This means the energy released when forming london forces does not exceed the energy required to break the hydrogen bonding in molecules = no solubility.
Carboxylic acids of up to 4 carbons are soluble in water because the energy required to break the hydrogen bonds in water is the same as the energy released when hydrogen bonds between the carboxylic acid and water form
What happens when methanol is oxidised?
technically it should go to a carboxylic acid (methanoic acid) but in practice the oxidation can go all the way to carbon dioxide and water
6 ways to prepare carboxylic acids?
1) oxidation of primary alcohols
2) oxidation of aldehydes
3) acid/alkali hydrolysis of nitriles
4) grignard + CO2
5) acid/alkali hydrolysis of esters (ester + water –> carboxylic acid + alcohol) OR (ester + alkali –> salt + alcohol then protonate salt to produce carboxylic acid)
6) acyl chloride + water –> carboxylic acid + HCl
Half equation for oxidation of dichromate ions (orange to green)
Cr2O7 2- + 14H+ + 6e- ———> 2Cr3+ + 7H2O
Conditions for preparing carboxylic acids from primary alcohols/aldehydes?
What is the other product?!
- potassium dichromate (VI) with sulphuric acid
- heat under reflux
- produces H20 as well!!!
full equation for primary alcohol to carboxylic acid (eg. ethanol)
CH3CH2OH + 2[O] —-> CH3COOH + H2O
2[O] because 2 lots of oxygen, one bit from to aldehyde and second bit to carboxylic acid
What does heating under reflux do and what do you do when the reaction is done and the carboxylic acid has formed?
Heating under reflux prevents any aldehyde formed from escaping before it has time to be converted to a carboxylic acid
Mixture distilled when reaction is complete (get aqueous solution of the acid)
Name two ways nitriles can be formed
halogenoalkane + cyanide ions
aldehyde/ketone + HCN (makes hydroxynitriles)
Name two ways to convert nitriles into carboxylic acids
which is usually better
- acid hydrolysis of nitriles (this bad boi is better)
- alkaline hydrolysis of nitriles
3 things about acid and alkali hydrolysis to prepare carboxylic acids
- heat under reflux
- dilute acid (eg. HCl) // alkali solution (eg. NaOH)
- distill off carboxylic acid from mixture once formed
Steps of alkaline hydrolysis of nitriles
NITRILE + WATER + ALKALI —-> SALT + AMMONIA
ethanenitrile —> sodium ethanoate (with ethanoate ions ofc) + NH3
CH3CN + H2O + OH- ———-> CH3COO- + NH3
Add HCl:
1) CH3COO- + H+ ——> CH3COOH (protonation)
2) NH3 + H+ ——> NH4Cl
4 things that acyl chlorides react with (spec)
- water
- alcohols
- concentrated ammonia
- amines
acyl chloride + water
carboxylic acid + HCl
acyl chloride + alcohol
ester + HCl
acyl chloride + concentrated ammonia
amide + HCl
acyl chloride + amine
secondary substituted amide + HCl
acyl chloride + water
knowledge
- hydrolysis reaction
carboxylic acid + alcohol
ester + water
Name the acid hydrolysis of esters equation
acid catalyst
ester + water –> carboxylic acid + alcohol (reversible so reaction does not go to completion)
(acid catalyst)
Name the alkali hydrolysis of esters equation
ester + alkali –> salt + alcohol
How are polyesters formed
dicarboxylic acid + diol
Four things carboxylic acids react with (spec)
1) lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether
2) bases to produce salts
3) phosphorus(V) chloride (phosphorus pentachloride)
4) alcohols in the presence of an acid catalyst
carboxylic acid + LiAlH4
- finish equation
- type of reaction
- what it forms
- general equation
- conditions
- what doesn’t work
——[4H]—-> alcohol + water
- reduction
- forms primary alcohol
- RCOOH + 4[H] —–> RCH2OH + H2O where R is H or hydrocarbon group
(4 H because LiAlH4 has 4 H) - LiAlH4(reducing agent) must be with dry ether because it is very reactive with water
- room temp
- end of reaction is complex aluminium salt, convert to alcohol by adding dilute sulphuric acid
- NaBH4 won’t work in this case as it is not a strong enough reducing agent
carboxylic acid + base
4 things
salt + water
- dilute ethanoic acid + NaOH (produces colourless solution of sodium ethanoate)
- temp of mixture increases
- ionic equation: H+ (aq) + OH- (aq) —> H2O(l)
- NOTE: base could also mean metal hydroxide
How can carboxylic acids be reduced to primary alcohols?
- react with LIAlH4
it’ll go from carboxylic acid — aldehyde —– primary alcohol
Naming esters
prefix - alcohol
suffix - carboxylic acid
………yl…….oate
eg. propyl methanoate
(formed from propanol and methanoic acid)
Drawing esters
the atoms from the alcohol connect to the single bonded O in the ester
propyl methanoate
(atoms from propanol would be connected to single bonded O)
carboxylic acid + alcohol
- finish equation
- type of reaction
- general equation
- conditions
ester + water
- esterification/reversible
- R-COOH + R’–OH ——-> R-COO-R’ + H2O
- acid catalyst (H2SO4)
- NOTE: R can be H, R’ cannot be H
two uses of esters
- flavouring
- used as solvents
How do you convert carboxylic acids into acyl chlorides?
react with PCl5
carboxylic acid + PCl5
- general equation
- observation
- state of products
——-> acyl chloride + HCL + POCl3
- R-COOH + PCl5 —-> R-COCl + POCl3 + HCl
- steamy white fumes
- liquid acyl chloride, phosphorus compound
-acyl chloride separated by fractional distillation
swapping -OH in -COOH for a -Cl
eg. ethanoic acid –> ethanoyl chloride
general background about acyl chlorides
very reactive, important because used to make a wide range of lots of other things
acyl chloride functional group
O
II
C - Cl
ester functional group
xxxxxO
xxxxxII
R—–C—–O—R’
acyl chloride + water
type of reaction
what replaces what
observation
conditions
MECHANISM
carboxylic acid + HCl
- Nucleophilic addition/elimination reaction
- Cl is replaced by -OH from the water
- steamy white fumes
- vigorous at room temperature
- cold water (if too high temp the carboxylic acid may dissociate and go on to other reactions)
see written flashcard
acyl chloride + alcohol
type of reaction observation conditions what is the nucleophile what is good about it
ester + HCl
- Nucleophilic addition/elimination reaction (Cl is eliminated and then H is eliminated and together they form HCl)
- steamy white fumes
- very exothermic reaction
- room temperature
- alcohol is the nucleophile
- quick and easy way to form an ester
acyl chloride + amine
N-substituted amide
- nucleophilic addition/elimination
- amine is the nucleophile
- concentrated solution of the amine
- violent reaction producing white solid mixture of N-substituted amide and (methyl)ammoniumchloride
XXX
acyl chloride + concentrated ammonia
reactant
aggressiveness
observation and what is produced
what remains
amide + HCl
- concentrated solution of ammonia in water
- very violent reaction
- mixture of solid ammonium chloride and ethanamide (lots of white smoke)
- Some of the mixture remains dissolved in water as a colourless solution.
XXX
What about aldehydes/ketones makes them susceptible to nucleophilic attack?
the C=O bond has a delta positive carbon which nucleophiles are attracted to
How do aldehydes and ketones differ in terms of oxidation?
aldehydes have a H atom attached to the carbonyl group which makes them very easily oxidised. Ketones do not have this so are resistant to oxidation and only very powerful oxidising agents that break C-C bonds can oxidise them
What happens to boiling point as aldehyde/ketone chain length increases?
- more electrons
- more and stronger london forces
- more temporary dipoles
Why do aldehydes and ketones have lower boiling points than alcohols?
- they cannot form hydrogen bonds
- because all their H atoms are attached to carbon atoms (no H atoms attached directly to the oxygen in the C=O)
(which means no delta positive H in one carbonyl for the O in C=O of another carbonyl to bond to)
How soluble are aldehydes and ketones?
- smaller ones are soluble because they can form hydrogen bonds with water
- but solubility decreases as chain length increases because the alkyl groups interrupt the water’s hydrogen bonds but replace them with only london forces so energy required to break the H bonds exceeds to energy released when the london forces formed hence they are not soluble)
How do carbonyl groups affect bonding?
They form polar bonds, because oxygen is much more electronegative than carbon
What intermolecular forces are present in a mixture of just ketones/a mixture of just aldehydes?
london forces
permanent dipole- permanent dipole (attraction between delta negative O of one ketone and the delta positive C of another ketone)
For ketones and aldehydes of a similar molecular mass, why do the ketones have a higher boiling point?
ketone’s carbonyl group is more polarised than an aldehyde’s so stronger permanent dipole-permanent dipole forces between ketone molecules to break
Why are small ketones more soluble than small aldehydes?
because of the distribution of alkyl groups around the C=O bond in ketones
What are the relative boiling points of aldehydes, ketones, alcohols and alkanes?
Why:
alcohol>ketone>aldehyde>alkane
Why:
alcohol - london, dipole-dipole, hydrogen bonds
ketone - london, stronger permanent dipole-permanent dipole than aldehydes
aldehyde - london, weaker permanent dipole-permanent dipole than ketones
alkanes - london
What does the iodoform reaction identify?
presence of a CH3CO group in an aldehyde/ketone
Explain the iodoform reaction and positive result
iodine and alkali (NaOH) added to ketone/aldehyde
CH3CO group present = yellow precipitate of CHI3
antiseptic smell
What types of aldehydes and ketones give a positive result for the iodoform reaction?
- methyl ketones (have a CH3 group attache dto one side of the C=O)
- only aldehyde is ethanal (has a CH3 group attached to one side of C=O - all other aldehydes do NOT)
What is the equation for the reaction between a carbonyl compound and iodine? (alkali)
CH3CO-R + 3I2 + 4OH- ——> RCOO- + CHI3 + 3I- + 3H20
How can Brady’s reagent (2-4dnph) be used to test for aldehydes and ketones?
An orange precipitate forms if a carbonyl is present
The precipitate can be purified by re-crystallisation
Each different carbonyl give a crystal with a different melting point
If the melting points are measured and compared to a table of known values, the compound can be identified
What is the reagent for reduction of aldehydes and ketones?
- LiAlH4 (reducing agent) in dry ether
- ketones are reduced to secondary alcohols
- aldehydes are reduced to primary alcohols
- [H]
How can you distinguish between aldehydes and ketones?
Aldehydes react with Benedict’s, Fehling’s and Tollens’, ketones do not
What reagents can be used to oxides aldehydes to carboxylic acids?
Acidified potassium dichromate, Fehling’s, Benedict’s, Tollens’
What is the colour change for Fehling’s/Benedict’s?
deep blue solution —> dark red precipitate
- heat gently
- ONLY aldehydes oxidised
Cu2+ reduced to copper (I) oxide
What is the colour change for Tollens’?
aldehyde present (ONLY aldehydes are oxidised)
- ammoniacal silver nitrate (silver nitrate, sodium hydroxide, ammonia solution)
- silver is reduced
- colourless solution –> silver mirror
Why can tertiary alcohols not be oxidised?
they have no hydrogen attached to the carbon with the OH group
Why can’t ketones be oxidised?
they have no singular H atom attached to the C=O
How are grignard reagents prepared?
- heat under reflux
- magnesium turnings with dry ether and halogenoalkane
- if reaction not immediate use crystal iodine as catalyst
- rub Mg with abrasive to remove oxide layer so halogenoalkane can directly react
two reactions of grignard reagents
grignard reagent _ CO2 —-> carboxylic acid
grignard reagent + aldehyde/ketone —-> alcohol
grignard reagent + CO2
carboxylic acid
1) bubble CO2 through grignard in dry ether
2) add dilute HCl to solution
*R from grignard attaches to C, double bond in CO2 breaks to form one single bonded oxygen, H from acid attaches to the single bonded oxygen, Cl from acid attaches to the MgBr (grignard) left over when the R detached)
R-MgBr + CO2 —> R-COOH + MgBrCl
grignard reagent + aldehyde/ketone
alcohol
1) bubble aldehyde/ketone through grignard in dry ether
2) add dilute HCl to solution
*R from grignard attaches to C, double bond in C=O breaks to form single bond, H from acid protonates by attaching to the now single bond O in C=O, Cl from acid attaches to left over MgBr (grignard) left over when the R detached)
How are polyesters formed?
Through condensation polymerisation
What are the physical properties of esters?
Colourless liquids with low melting and boiling points, insoluble in water, no hydrogen bonding. Generally pleasant smell.
explain alkaline hydrolysis of esters
- conditions
- advantages over acid hydrolysis
- methyl propanoate example word and chemical equation
- how would you form a carboxylic acid from this?
ester + alkali –> salt + alcohol
- ester heated under reflux with alkali (usually NaOH)
- advantages over acid hydrolysis: reaction one way rather than reversible, products easier to separate
- methyl propanoate —-> sodium propanoate + methanol
- CH3CH2COOCH3 + NaOH —> CH3CH2COO-Na+ + CH3OH
- add a strong dilute acid (HCl) which will protonate the propanoate ions (or whatever they are) in the salt. Carboxylic acid then distilled off.
explain acid hydrolysis of esters
- conditions
- how to make reactions as complete as possible
- example word and chemical equation with methyl propanoate
ester + water –> carboxylic acid + alcohol
- ester heated under reflux with dilute acid (HCl acts as catalyst)
- reversible - to make reaction as complete as possible use excess dilute acid which contains excess water needed
- methyl propanoate —> propanoic acid + methanol
- CH3CH2COOCH3 + HC20 —–(H+)—-> CH3CH2COOH + CH3OH
How are hydroxynitriles formed?
carbonyl + HCN
What is a hrydroxynitrile?
has an -OH and a -CN
carbonyl + HCN
hydroxynitrile
- nucleophilic addition
- room temp + pressure
- HCN (g) v poisonous so used in solution with KCN (and little sulphuric acid)
- nucleophile = cyanIDE ion (:CN-)
- HCN adds across the double bond
- needs both HCN and :CN- in order to work
- pH 4-5 gives fastest reaction
- H added may also come from H20 or H30+ in slightly acidic solution
How does this equilibrium
HCN H+ + CN-
affect the carbonyl + HCN mechanism?
the more acidic, the more HCN associates and equilibrium shifts left and so fewer CN-
reaction will be slower because the first step requires :CN- to act as the nucleophile.
Explain whether a ketone or aldehyde reacting with HCN would give an optically active mixture as a product?
ketone:
once the CN has added on, it has 4 DIFFERENT groups around the chiral carbon.
original ketone was planar = nucleophile could attack from either side = racemic mixture = NOT optically active
original ketone was NOT planar = nucleophile only attacked from one side = optically active mixture forms
Aldehyde:
When :CN- adds it can never form a product with 4 different groups around a chiral carbon = never produces an optically active product
What is beneficial about a slightly acidic solution in the mechanism reacting carbonyls and HCN?
additional H+ increase polarity of C=O bond by attracting e- more strongly to oxygen end
so C more delta negative and nucleophile more attracted
explain the carbonyl + HCN mechanism really simply
- :CN- attacks delta positive C in C=O of carbonyl
- C=O becomes C-O
- lone pair on O attracted to H in H-CN (takes H away and forms OH bond)
- hyroxynitrile + :CN-
2 ways of producing esters
1) acyl chloride + alcohol –> ester + HCl
2) carboxylic acid + alcohol —> ester + water
all the reactions (general)
- acid/alkali hydrolysis of nitriles
- acid/alkali hydrolysis of esters
- 4 acyl chloride reactions
- 4 carboxylic acid reactions
- 2 grignard reactions
- 2 reactions to produce nitriles
- 2 oxidation reactions to produce carboxylic acids
