Aromatic Compounds, Carbonyls and Acids Flashcards
What is benzene?
A naturally occurring aromatic compound which is a very stable planar ring structure with delocalised electrons.
What did Kekulé suggest was the structure of benzene?
Benzene was a six-membered ring with alternating single and double bonds between carbon atoms.
What were the three problems with Kekulé’s model?
1) Benzene is resistant to addition reactions.
2) Enthalpy of hydrogenation of benzene shows it to be more stable than predicted.
3) A six bonds carbon bonds are the same length.
What does the delocalised structure of benzene show its structure to be?
Each of the 6 carbon atoms donates one electron from its p-orbital. These electrons combine to form a ring of delocalised electrons above and below the plane of the molecule. Electrons are said to be delocalised as they do not belong to a single atoms. All bonds are the same length.
What is a substitution reaction?
Where a group or atom is exchanged for another group or atom in a chemical reaction.
What is a benzene derivative?
A benzene ring that has undergone a substitution reaction.
What is electrophilic substitution?
A substitution reaction where an electrophile is attracted to an electron rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting an electron pair.
What is a reaction mechanism?
A model with steps to explain and predict a chemical reaction.
What is a Friedel-Crafts reaction?
A substitution reaction where hydrogen is exchanged for an alkyl or acyl chain.
What are phenols?
A class of aromatic compounds where a hydroxyl group is directly attached to the aromatic ring.
Why is phenol more reactive than benzene?
The p-orbital electrons from the oxygen of the hydroxyl group add to the delocalised electrons of the aromatic ring. So the pi system becomes more nucleophilic. This increase in electron density allows the aromatic ring in phenol to be more susceptible to electrophilic attack as it can induce a dipole on non-polar molecules.
What is a nucleophile?
A species attracted to an electron-deficient part of a molecule, where it donates a pair of electrons to make a new covalent bond.
How do you test for a carbonyl functional group (ketone or aldehyde)?
Add Brady’s reagent and a yellow/orange precipitate is formed if a carbonyl functional group is present.
How do you test for an aldehyde?
Add Tollen’s reagent to it and if a silver mirror is observed, an aldehyde is present.
How does solubility change as hydrocarbon chain length of carboxylic acids increases?
The solubility decreases because only the polar carboxylic acid functional group can form hydrogen bonds with water so as more of the molecule becomes non-polar.
