Topic 17 Organic 2 Flashcards
What is structural isomerism?
When compounds have the same molecular formula but different structural formulas.
E.g. both have C₃H₇Cl
but one is CH₃CH₂CH₂Cl and the other is CH₃CHClCH₃
What is stereoisomerism?
Where molecules have the same structural formula but the atoms are arranged differently in space.
This consists of both E/Z (Cis/Trans) isomerism and optical isomerism.
What is chain isomerism?
Where structural isomers have different arrangements of the carbon skeleton.
E.g. 2-methylbutane is a chain isomer of pentane.
What is position isomerism?
Where structural isomers have the same functional group attached to the same carbon chain, but in a different position.
E.g. butan-2-ol is a position isomer of butanol.
What is functional group isomerism?
Where structural isomers have different functional groups.
E.g. butanol is a functional group isomer of butane.
What is Cis/Trans isomerism? What is E/Z isomerism?
Cis/Trans - These are terms used to describe the structure of isomers. Cis is used if both the functional groups attached to each carbon in a C=C are on the same side. Trans is used if both the functional groups attached to each carbon in a C=C are on different sides.
E/Z - This is an extension of Cis/Trans isomerism that is used for more complex molecules. E is entgegen (opposite) and is the same as Trans. Z is zusammen (same) and is the same as Cis. The more complex molecules usually have more than one functional group attached to a C=C and therefore the functional group with the highest priority (highest atomic number) is used to identify the molecule as E/Z.
What is a chiral molecule?
A molecule that has a non-superimposable mirror image. This is due to the structure of the two molecules.
E.g. Human hands are mirror images of each other but are non-superimposable.
What does superimposable mean?
The ability to place two mirror images on top of each other to make two identical products without rotating/changing either object.
E.g. Hands are mirror images of each other and match if one hand is rotated and placed on top of the other. However, because a hand needs to be rotated, they are not superimposable.
What is an asymmetric carbon atom?
A carbon atom that is attached to 4 different functional groups.
E.g. CH₃-CH(OH)-COOH
The carbon atom in CH(OH) is attached to the CH₃, H, OH and COOH functional groups.
What is an asymmetric molecule?
A molecule that has no centre, axis or plane of symmetry. All asymmetric molecules are chiral.
What is an optical isomer/enantiomer?
When a chiral molecule and its non-superimposable mirror image rotate the plane of plane-polarised light in two different directions (one clockwise and the other anticlockwise).
What is a racemic mixture?
A mixture of equal amounts of the two mirror-image forms of a chiral compound. The mixture does not rotate polarised light as the two optical isomers have equal and opposite effects so they cancel out.
What is a nitrile?
An organic compound with the C≡N functional group.
E.g. Propionitrile
What is nucleophilic addition?
When an electron-rich species, a nucleophile, attacks the electron-deficient carbon atom of the unsaturated C=O bond, leading to the formation of a C-O single bond.
What is a nucleophile?
An electron-pair donor.
What is a crystalline derivative?
A chemical which can be purified by recrystallisation and then identified by measuring the melting temperature.
What is an aldehyde?
An organic compound with the CHO functional group.
This is different from the COH functional group (alcohol) as it contains a C=O bond and C-H bond instead of one C-OH bond.
E.g. Propanal (ane replaced by al)
What is the carbonyl group?
A group of organic functional groups which contain the C=O bond. Carbonyl compounds such as aldehydes and ketones only have carbon or hydrogen bonded to the C=O bond.
Other functional groups in the carbonyl group that contain the C=O bond but have other atoms then carbon and hydrogen attached to the C=O bond are not carbonyl compounds. These consist of carboxylic acids, acyl chlorides, esters and amides.
What is a ketone?
An organic compound with the C=O bond in the middle (with 2 carbons attached to the carbon in the C=O bond).
E.g. Prop-2-one
How are carbonyl compounds (aldehydes and ketones) formed?
Aldehydes are formed by the oxidation of a primary alcohol. Ketones are formed by the oxidation of a secondary alcohol. For both, an excess of alcohol is distilled with potassium or sodium dichromate (VI) and dilute sulphuric acid.
How can carbonyl compounds (aldehydes and ketones) be oxidised?
Aldehydes can be oxidised to form a carboxylic acid. For this, acidified potassium dichromate (VI) is added to the aldehyde and the mixture is distilled under reflux. To identify when the carboxylic acid has been formed, there will be a colour change from orange (due to the initial Cr₂O₇²⁻ ions) to green (due to the Cr³⁺ ions).
Ketones can be oxidised further through the use of very strong oxidising agents.
What does heating under reflux mean?
This means that no reactant (aldehyde) is removed during the heating process. This is done by the use of a reflux condenser which condenses any gases back into the mixture.
In the case of oxidising aldehydes, due to the difference in boiling points between the aldehyde and carboxylic acid, a clear colour change can be observed when all the aldehyde has been oxidised as the carboxylic acid does not evaporate.
How can carbonyl compounds (aldehydes and ketones) be reduced?
Metal hydrides can reduce carbonyl compounds to alcohols. Lithium tetrahydridoaluminate (LiAlH₄) is a powerful reducing agent that can reduce aldehydes to primary alcohols and ketones to secondary alcohols.
How can aldehydes and ketones be distinguished?
Because aldehydes can be oxidised with mild oxidising agents but ketones cannot, three mild oxidising agents can be used to distinguish between an aldehyde and a ketone. These are Fehling’s Solution, Benedict’s Solution, and Tollens’ Reagent.
