Chapter 17 - Organic chemistry 2

What is a stereoisomer?

An isomer with the same structural formula but a different arrangement in space.

What is an optical isomer?

A type of stereoisomer where two molecules are object and non-superimposable mirror images of each other (also called enantiomers). This is the result of chirality in molecules with a single chiral centre.

What is a chiral carbon?

A carbon atom with 4 different groups bonded to it (asymmetrical molecule) meaning that there are two possible ways or organising the atoms or groups giving 2 possible stereoisomers. This chiral carbon is the chiral centre of the molecule.

How should you draw optical isomers?

• identify the chiral centre

• Once you know the chiral centre, draw one enantiomer in a tetrahedral shape.

• Don't try to draw the full structure of each group, just write their symbols

• Then draw a mirror image beside it.

• (If there's more than one chiral centre, mirror each chiral centre one by one to get all the possible isomers.)

What is optical activity?

This is how one optical isomer will rotate plane-polarised monochromatic light in an opposite direction to its isomer. One isomer rotates plane-polarised light clockwise (+). The other rotates plane-polarised light anticlockwise (−).

What is plane polarised monochromatic light?

Plane-polarised monochromatic light is light of only one wavelength that vibrates in one direction only

What is a racemic mixture?

A mixture that contains equal amounts of each enantiomer of a chiral compound.

What is the optical activity of racemic mixtures and why?

A racemic mixture is optically inactive as the enantiomers will cancel out each others effect. This means that the plane of polarised light will not change

How can racemic mixtures be formed?

Racemic mixtures often form when a reaction involves an intermediate that is planar, such as in the SN1 mechanism. This allows the nucleophile to attack from either side with equal probability.

What can optical activity be used for?

The optical activity of product mixtures can give clues about the type of mechanism that a reaction follows, especially for SN1 and SN2 reactions (types of nucleophilic substitution) .

What is a Sn1 reaction?

• The S_N1 mechanism is a two-step reaction

  • In the first step, the C-X bond breaks heterolytically and the X leaves the enantiomer as an X^- ion

  • This leaves a trigonal planar, tertiary carbocation 

  • In the second step, the planar, tertiary carbocation is attacked by the nucleophile

  • The nucleophile is able to attack from either side of the planar carbocation with equal probability, which results in the formation of a racemic mixture

• Therefore, a reaction with an S_N1 mechanism will produce a racemic mixture

• If a product mixture is optically inactive but formed from an optically active compound, it suggests an SN1 mechanism.

What would the mechanism for a Sn1 reaction look like ?

What is a Sn2 reaction?

• The S_N2 mechanism is a one-step reaction

  • The nucleophile donates a pair of electrons to the δ+ carbon atom of the enantiomer to form a new bond

  • At the same time, the C-X bond is breaking and the X takes both electrons in the bond 

  • The X leaves the enantiomer as an X^- ion

• This produces only one optical isomer, meaning the product mixture is optically active.

What would the mechanism for a Sn2 reaction look like ?

What is a carbonyl compound?

Carbonyl compounds are organic molecules that contain a carbonyl group, which is a carbon atom double-bonded to an oxygen atom (C=O)

What is an aldehyde?

Aldehydes are carbonyl compounds with the carbonyl group at the end of the carbon chain.

Their names all end in -al and 1 is not needed in the name as all will have this group at the end of the chain

They will all have an H bonded to the C in the C=O

What is a ketone?

Ketones are carbonyl compounds with the carbonyl group in the middle of the carbon chain.

Their names end in -one and they usually need a number to describe where in the chain it is.

What is the boiling points of aldehydes and ketones like in comparison to their alcohol counterparts?

They have lower bp and mp than alcohols as they are unable to make hydrogen bonds with themselves as they don’t have a polar O-H bond. They can instead form dipole dipole (polar C=O bond) and London forces.

Are Ketones and Aldehydes soluble in water?

They have lone pairs on their oxygen meaning that the oxygen can use its lone pairs to form hydrogen bonds with hydrogen atoms on water molecules (The δ– oxygen atom from the carbonyl uses its lone pairs to form hydrogen bonds with the δ+ hydrogen from water) Therefore small aldehydes and ketones will dissolve in water.

Larger aldehydes and ketones have longer hydrocarbon chains which cannot hydrogen bond with water. These hydrocarbon chains can disrupt the hydrogen bonding within water but cannot form hydrogen bonds themselves

So if an aldehyde or ketone is large enough, the intermolecular forces (in this case London forces) between the aldehyde or ketone molecules, and the hydrogen bonding between water molecules will be stronger than the hydrogen bonds that could form between the aldehyde/ketone and water. So the compound won't dissolve. (In order to dissolve, the strength of the potential hydrogen bonding of the carbonyl with water must be higher than the combined strength of the intermolecular forces of the carbonyl and the hydrogen bonding of water)

Solubility decreases with chain length.

How do aldehydes and ketones react with Acidified potassium dichromate(VI)?