Module 6: Section 1 Flashcards
What is the general formula for benzene?
C6H6
What was Kekule’s model of benzene?
Cyclic structure, with 6 carbon atoms joined in a ring, with alternating single and double bonds.
What 3 factors proved Kekule’s model of benzene to be wrong?
- Enthalpy change when benzene is hydrogenated is considerably lower than we would expect, it is less exothermic than we expect
Suggests benzene has a stable structure - not alternate single and double bonds. - When benzene is analysed, we only get 1 bond length.
Suggests all bonds are the same length, an intermediate between a single and double bond. - Benzene is far less reactive than we expect.
Suggests a stable structure.
Describe the delocalised model of benzene.
P-orbitals of the 6 carbon atoms overlap to create a pi system. The pi system is made up of 2 ring-shaped clouds of electrons, one above and one below the plane of the molecule. All bonds in the ring are the same length. Electrons in the ring are delocalised as they dont belong to a specific carbon atom.
Why doesnt benzene like to react by electrophilic addition?
In benzene you have a delocalised electron ring above and below the plane of the molecule. They spread out the negative charge and make the benzene very stable. Benzene is unwilling to undergo addition reactions, as they would destroy the stable ring.
How does Benzene prefer to react?
Name the mechanism
Benzene prefers to react by electrophilic addition.
How do halogen carriers help halogens substitute into the benzene ring?
To attack the benzene ring, an electrophile must have a strong positive charge. Compounds can be made into stronger electrophiles using a catalyst called a halogen carrier.
Halogen carriers accept a lone pair of electrons from a halogen atom on an electrophile. As the lone pair of electrons is pulled away, the polarisation in the molecule increases and soemtimes a carbocation forms.
Describe the structre of phenol, and give the general formula.
Benzene ring with OH attached.
Formula: C6H5OH
Why is phenol more reactive than benzene?
One of the lone pairs on the oxygen atom overlaps with the delocalised ring of electrons. Lone pair of electrons from the pxygen atom is partially delocalised into the pi system. This increases the electron density of the ring, so is more likely to be attcaked by electrophiles.
Describe an aldehyde group.
Aldehydes have their carbonyl group at the end of the carbon chain. Their names end in -al.
Describe a ketone group.
Ketones have their carbonyl group in the middle of the carbon chain. Their names end in -one
What do you produce when you oxidise primary alcohols?
When primary alcohols are distilled, you produce an Aldehyde.
When primary alcohols are refluxed, you produce a carboxylic acid.
What is produced when you reduce:
- Aldehydes.
- Ketones.
- Reduce an aldehyde to a primary alcohol.
- Reduce a ketone to a secondary alcohol.
Reducing agent [H] is used.
What test is used to determine the carbonyl compound?
2-4 DNPH (Bradys reagant) is dissolved in methanol and conc sulphuric acid.
Reacts with carbonyl compunds to form bright orange ppt.
Measure the mpt of the crystals and compare against known mpts to identify the carbonyl comnpund.
Used to identify Ketones and Aldehydes.
How do you test for aldehydes?
Silver mirror test.
Tollens reagant is used.
When heated, the aldehyde is oxidised and silver ions in the tollens reagant are reduced.
Causes a silver mirror to form.
Distinguish between aldehydes and ketones, as an aldehyde will be easily oxideised to a carboxylic acid, but a ketone wont.
