Topic 17 Organic II Flashcards
What are 2 types of isomerism?
structural
stereo
what are 3 types of structural isomerism?
- chain
- position
- functional group
what are 2 types of stereoisomerism?
- geometric
- optical
What are optical isomers?
- same molecular
- same structural
- different 3D spatial arrangement
what are conditions needed for geometric isomers?
double C=C bonds as it limits rotations
What are conditions needed for optical isomers?
- must have chiral centre
(usually C) - Has 4 (or more) different groups bonded to it
What does nonsuperimposable mean?
Like hands
have the same properties and structure
but are mirror images of each other
What can make a racemic mixture?
- SN1 (sometimes)
- MUST have chiral centre
No rotation when plane polarised light is shown through…
think RACEMIC MIXTURE
For sure
what is the only difference between enantiomers?
rotate plane polarised (monochromatic) light in equal but opposite directions
why do enantiomers rotate monochromatic lights in different directions?
due to their different arrangements of atoms in space
why must it be racemic mixtures when there are no rotations through polarised light?
enantiomer rotations cancel out
How are enantiomers / racemic formed?
50/50 chance for SN1 attack of chiral centre (from above or below trigonal planar plane)
other bonds pushed up or down when new bond formed
forming enantiomers in equal portions
What can be used to test for carbonyls?
- dichromate
- Benedict’s/fehlings
- tollens (ammoniacle silver nitrate)
- Lithium aluminium hydride (LiAlH4)
What is dichromate’s reaction to carbonyls? +why?
Aldehyde: Orange to green as aldehyde is oxidised to -COOH
Ketone: remain orange, no change
Can ketones be oxidised or reduced? How?
Only reduced
By LiAlH4 in dry ether
What is the Benedict’s/Fehling’s reactions to carbonyls? WHY?
Aldehydes: Blue to brick red ppt
Cu 2+ + Aldehyde -> Cu2O (Cu2+ is reduced)
No change in ketones
Aldehyde + alkaline oxidising agent ->
Carboxylate + water
How to turn carboxylate into carboxylic acid?
Add strong acid
To Protonate into carboxylic acid
What is tollens reagent’s reaction to carbonyls?
Aldehydes: Silver mirror formed - Ag (s) from Ag+ , Ag+ reduced
No change in ketone
What does LiAlH4 stand for?
Lithium aluminium hydride
What is LiAlH4’s reactions to carbonyls?
In dry ether
Acts as reducing agent - [H]
Aldehyde + [H] → 1° alcohol
Ketone + 2[H] → 2° alcohol
What is the the reaction equation of Fehling’s solution with aldehyde? And explain colour change
2 Cu2+ (aq) + 2OH- (aq) + 2e- -> Cu2O (s) + H2O (l)
Hence blue -> red
What is tollen’s reagent’s reaction with aldehyde? Give equation
And what change is observed?
2Ag+ (aq) + 2OH- (aq) -> Ag2O (s) + H2O (l)
Why can’t tertiary alcohols or ketones be oxidised?
They must have an H atom bonded to C to be oxidised
What is [H]?
Reducing agent
What is formed when carbonyls + HCN?
hydroxy nitrile
What does HCN stand for and is it a strong or weak acid?
Hydrogen cyanide
Weak
What catalyst can be added to the carbonyl + HCN reaction to speed up RoR?
KCN (salt)
It provides loads of cyanide nucleophiles
(In this case conc of catalyst impacts RoR)
Are carboxylic acids strong or weak acid?
Weak
How to test for carboxylic acids?
Add sodium carbonate (Na2CO3 / NaHCO3)
- fizzing to indicate CO2 produced
What’s the best indicator to use to test for carboxylic acids?
Universal indicator
Or pH probe
How soluble is carboxylic acids in water? Why?
- very!
- when R group is small, H bonds can form
Does solubility for carboxylic acids increase or decrease as chain length increases? Why?
Carb. aciDDDDs so DDDDecreases
As non polar part of molecule is more significant
What’s a dimer in terms of carboxylic acids
2 of them bonded together covalently (=O - - - HO-)
Therefore explains high mp/bp than expected
What are 3 methods to make carboxylic acids?
- Distill 1° alcohol with [O] like K2Cr2O7
- Reflux 2° alcohol with [O]
- Hydrolysis of nitriles
What are 2 methods of hydrolysis of nitriles to make carboxylic acids?
+ water
1. Acid catalysed
2. Alkali catalysed, makes carboxyLATE then protonate by adding strong acid
How to make carboxylic acids via hydrolysis using acids?
nitrile + water -H+—> carboxylic acid + ammonium
How to make carboxylic acids using nitriles and alkali?
Nitrile + water + OH- —> carboxyLATE + ammonia
Then protonate by adding STRONG acid to make
How to turn carboxylic acid into aldehyde?
+ dry ether and reflux in LiAlH4
+ dilute acid
makes 1° alcohol
+ oxidising agent
into aldehyde
what are examples of good bases?
metal oxides/hydroxide/carbonates/hydrogencarbonates
ammonia/ammonium hydroxides
What does monoprotic mean in terms of -COOH?
they make salt and water when reacted with bases
what does PCl5 test for?
-OH groups
PCL5 + -COOH ->
RCOCl + HCl (g) + POCl3
white misty fumes (HCl)
-oyl chloride
- v rigorous reaction
carboxylic acid + alcohol (+ H+) ⇌
ester + water
e.g. ethyl methanoate
what is the ester functional group?
C = O
|
O
what is the acyl chloride functional group?
Cl – C = O
COCl
how to make acyl chlorides from carboxylic acids?
+ PCl5
are acyl chlorides reactive and why?
v reactive
since the C is more δ+ (than in -COOH)
it is bonded to two v δ- atoms
pulling e- density away
therefore very easily attacked by nucleophiles
acyl chloride + water →
in what conditions?
HCl + carboxylic acid
keep acyl chloride in DRY ether as v reactive
acyl chloride + alcohol →
what kind of reaction is it?
ester + HCl
condensation & exothermic reaction
what are 2 methods of making esters?
-COCl + -OH
-COOH + -OH
compare the 2 ways of making esters
using COCl / COOH, both + -OH
goes to completion / doesnt
no heating / lots of energy needed to heat
corrosive products / not
acyl chloride + ammonia →
ammonium chloride + amide
NH4Cl + R-C(=O)(-NH2)
what is observed when acyl chloride + ammonia?
white smoke from NH4Cl
acyl chloride + 1° amine →
2° amine + HCl
e.g. N-methylethanamide
acyl chloride + 2° amine →
3° amide + HCl
e.g. N,N-disubstitutedamide
acyl chloride + 3° amine →
does not react
as no H on N atom to make HCl
amine vs amide?? (functional groups)
amine: -NH2
amide: -C(=O)(-NH2)
ester + water →/⇌
carboxylic acid + alcohol
→ in general
⇌ when acid catalysed and heat
ester + hydroxide →
carboxylate + alcohol
OH- is not a reactant! and alkaline not reversible.
Compare methods of esterification between RCOCl and RCOOH
With RCOCl / RCOOH
- reaction is irreversible / reversible
- HCl is by product / water
- reaction is very fast, acid catalyst not needed / slower, need acid catalyst
Do carbonyls or alcohol have higher or lower mp/bp (same number of C)
Why?
Carbonyls have much lower mp/bp
As no H bonds
And fewer e-s
What is 2,4-DNPH reaction to carbonyls?
Orange to bright yellow ppt
How to identify exact carbonyl with 2,4-DNPH?
(3 marks)
Filter bright yellow ppt from positive result
Recrystallise
Melt purified solid
Mp will tell identity of carbonyl
How to tell if 2,4-DNPH ppt is aldehyde or ketone?
If can be oxidised = aldehyde
What is Iodine in presence of alkali used to test
triiodomethane (CHI3) reaction can be used to identify the presence of a CH3CO group in aldehydes and ketones.
What is triiodomethane formula and what does it smell like?
CHI3
Hospitals
When can alkaline iodine + carbonyl/alcohol react?
What is formed?
Have CH3CO group
Forms bright yellow ppt CHI3
How to balance equation of iodine in presence of alkali
One organic
3 of everyone else
One extra NaOH
RCOCH3 + 4NaOH + 3I2 -> RCOONa + HCI3 + 3H2O + 3NaI
Define isomerism
Molecules with same molecular formula
But atoms are bonded in different ways to produce different compounds with different chemical and physical properties
Define chiral centre
An atom (often carbonyl with 4 diff groups bonded to it
Define structural isomer
Molecules w same molecular formula but different structural formulas
Define stereoisomerism
Molecules w same molecular and structural formula
But diff 3D arrangement of atom
Which ONE test tests for carbonyls?
2,4-DNPH
What organic compound smells like pineapples?
Esters
(If in doubt, go for esters)
What are the nomenclature 1-10
Meth Eth prop but pen hex hept oct non dec
Instead of saying Brady’s reagent, what is it’s official name? and what does it test for? What are the conditions and result?
2,4-DNPH
carbonyls
( - must have CH3CO group to react with iodine)
into bright yellow ppt
when propanone reacts with iodine in the presence of NaOH, the crystalline solid product has the formula…
What does it look like?
CHI3
triiodomethane
pale yellow
What conditions are needed to make racemic mixtures?
has chiral centre
w 4 different groups bonded to it
what is a molecular formula?
formula of compound but no structural info
e.g. C5H12O - could be an alcohol or ketone
An alcohol can be converted into a ketone by oxidation.
Explain why refluxing the mixture first, rather than immediately distilling the product over from the beginning results in higher yield of ketone.
(1 mark)
- so no volatile alcohol reactant will be lost
- ensure all alcohol oxidises
explain in terms of intermolecular forces in liquids why bp is
ethanal < ethanol < ethanoic acid
(3 marks)
- ethanoic has the most e-s so most london forces
- both ethanol and ethanoic acid can form H bonds between itself
- ethanoic acid can form dimers covalently
- so most energy needed to overcome
Why is KCN necessary to form hydroxynitriles with HCN + aldehyde? (1 mark)
KCN fully ionises to produce loads of CN- nucleophiles
so RoR increases
explain why the hydroxynitrile is not optically active. (2 marks)
The CN- nucleophile can attack trigonal planar with equal probability from above and below to form enantiomers
forming racemic mixture
each rotates pp light in equal but opp directions so they cancel out
what does optically active mean?
capability of rotating the plane of vibration of polarized light to the right or left
what are the 2 types of reactions to form a polymer?
condensation
or
addition (alkenes)
how are esters formed?
between 2 OH ends of molecule
Give a reason why 3-methylbutanoic acid cannot be formed from oxidising 3-butan-2-one from 3-methylbutan-2-ol.
Ketones cannot be further oxidised
Only primary alcohols can be oxidised to carboxylic acid.
State what it is meant by chiral. (1 mark)
Forms 2 superimposable isomers
Has 4 different groups attached to it
If something reacts with carbonates to produce CO2, what does it have to be?
An acid! (as water and salt also formed)
What reaction involves nucleophilic addition?
ALDEHYDE OR KETONE
+ HCN + KCN
MAKES RCNOH
