Edexcel Chemistry: Organic IIWatch
2. Carbonyl compounds
3. Carboxylic acids and derivatives
4. Chromatography and NMR
(This is not just for Edexcel!)
in the uk its going above 10,000 per day as a minimum
Chirality and enantiomers
Chirality is an atom which has four different groups connected to the central carbon atom, which is called as the chiral centre. It is an asymmetric carbon atom.
For example, CH3CCl2CH3 has no chiral centres as each carbon atom is connected to either 2 H or 2 Cl atoms.
You need to draw a 3D structure to analyse optical isomers, and you only need to exchange 2 opposite bonds to the left and right to find the optical isomer of a structure. An optical isomer is basically a reflected image of a mirror of that molecule.
2. How to prove with optical activity
Before talking about this, we should know that plane polarised light has oscillations in 1 plane at a right angle to the plane of travel. A polariser can be used to analyse it, as the light is converted to be polarised, before going into a sample of the substance.
A racemic mixture is an equimolar mixture (composition 50:50) of 2 optical isomers (or enantiomers) that have no optical activity. The laevorotatory isomer (rotated anti-clockwise) and the dextrorotatory isomer (rotated clockwise) have to be in the same angle of rotation.
1. Types of compounds
There are aldehydes and ketones. Aldehydes have an H atom joined to a carbonyl (C=O) group, with a suffix (-al). The general formula is RCHO. Ketones have only CH3, CH2CH3 and so on, which is hydrocarbon groups joined to C=O. They have suffix -one and need at least 3 carbons, with a general formula RCOR. There is a polar pi bond in both due to the difference in electronegativity between O and H.
The carbonyl group can be tested with 2,4-dinitrophenylhydrazine. Just after adding it to an aldehyde or ketone, it starts turning colour from green, eventually forming an orange precipitate. Here is the reaction:
The boiling points are the same at 102C, so they cannot be separated by boiling point, but with the precipitate (with a clear orange colour) formed, which is a solid, with a sharp melting point, which is easy to be filtered because it has no impurities. The melting points of precipitates formed with different aldehydes/ketones are different, so they can be separated with a melting point test.
They consist of a carbonyl and a hydroxyl group on the same C. The group is called -COOH, with a general formula RCOOH.
The reactions involve the loss of an H+ ion, forming a carboxylate ion. As there are two negative dipoles on the oxygen atoms, the double (pi) bond can be formed in either oxygen through the transfer of electrons.
They also have high boiling temperatures, because they have an O-H bond with dipole-dipole interactions. It can also have extensive hydrogen bonding, forming a dimer. They are soluble in water as they can form a maximum of two hydrogen bonds with water molecules. As the significance of R group is larger with a longer chain, O-H is less significant. This causes boiling temperature to increase and solubility in water to decrease with a longer chain.
Carboxylic acids have a COOH group
It has a C=O functional group and OH acid group
It allows H bonds to form between each other
It has much higher melting boiling points
H bonds with water shows it’s soluble
A reducing agent of LiAlH4
Provides H minus nucleophile, in a dry ether solvent
It’s immediately reduced to primary alcohols
Aldehydes can’t be isolated as they’re more reactive than acids
Carboxylic acids are a weak acid
And as it has a covalent nature, it reacts with a base
To produce ionic salt, in neutralisation
It is a type of carboxylic salt, formula is RCOO-
When OH replaced by Cl, it’s COCl
Must be anhydrous as both sides can react with water
Vigorous means there’s no heat, POCl3 must distill in a fraction
When it reacts with alcohols in H2SO4
An ester forms in reaction, it’s slow, reversible
Acid can be prepared by oxidation of alcohols or aldehydes
Or by hydrolysis of nitriles when the CN bond is broken
It needs dry ether in an acidic hydrolysis
Plus 1 H plus, two H2O, ammonium’s also here (direct form!)
In alkaline (hydrolysis) add an OH and also a H2O
Carboxylate ion’s formed, plus H+ it turns to carboxylic acid