Chapter 18 Flashcards
What is Benzene?
A compound with the formula C6H6 which has a cyclic structure with its six carbon atoms joined together in a ring
What are the two ways of representing Benzene?
The Kekule model and the delocalised model
What is the Kekule model of Benzene?
Each carbon atom was bonded to one hydrogen atom and the molecule was constantly flipping between 2 isomers by switching over the double and single bonds
What was the biggest problem with the Kekule model?
X-ray diffraction studies showed that all the carbon-carbon bond lengths were the same, which went against the idea of single and double bonds
In the delocalised benzene model, how many sigma bonds does each carbon form?
3, meaning each carbon has 1 remaining p-orbital
What do the remaining p-orbital from each carbon in benzene do?
They overlap sideways, to form a ring of pi-bonds which make up to clouds above and below the plane of carbon atoms. The electrons in these clouds are delocalised and do not belong to a specific carbon atom
How did the enthalpy change of hydrogenation provide more evidence for the delocalised model of benzene?
More energy was required to break the bonds in Benzene than the kekule model predicted, due to the extra stability of benzene, due to the delocalised ring of electrons
How do alkenes react with bromine?
They react easily with bromine water via electrophillic addition, which decoularises the bromine water from brown to colourless. The bromine atoms are added across the C=C double bond.
Why is benzene unwilling to undergo electrophillic addition reactions, unlike benzene?
The delocalised ring spreads out the negative charge, making benzene very stable, and unwilling to undergo addition reactions and destroy the stable ring
What mechanism does benzene prefer to react by?
Electrophillic addition
How does Benzene burn?
With a smoky flame, producing carbon dioxide and water, and a lot of the carbon stays as soot.
What are aromatic compounds?
Compounds containing a Benzene ring
How does benzene react with an electrophile?
Via an electrophilic substitution reaction
Benzene breaks the delocalised ring to react with the electrophile, forming an unstable intermediate, where the C-H bond breaks restoring the benzene ring
How can an electrophile be made into a stronger electrophile?
Using a halogen-carrier catalyst, which accepts a lone pair of electrons from a halogen atom on an electrophille, making the electrophile stronger and able to react with the benzene ring
How do you form bromobenze?
React benzene with Br2 and a halogen carrying catalyst (e.g. FeBr3) which pulls a lone pair of electrons from the reacting bromine atom to make the other bromide atom a stronger electrophile
What does fiedel-craft alkylation do?
It puts any alkyl group onto benzene using a halogenoalkane and a halogen carrier catalyst`
What does fiedel-craft acylation produce?
Ketones
How do you produce a phenylketone?
You reflux benzene with an acyl chloride
What carbocation is formed from the acyl choride and the AlCl3?
AlCl4- + CH3CO+
How do you form nitrobenzene?
Heat benzene with nitric acid, using sulfuric acid as a catalyst.
When forming nitrobenzene, what is the reaction between nitric acid and sulfuric acid?
H2SO4 + HNO3 –> HSO4- + H2NO3+
H2NO3+ —> NO2 + + H20
What is phenol?
A benzene ring with the -OH group attached
Why is phenol more reactive than benzene?
One of the lone pairs from the oxygen atom overlaps with the delocalised pi-bonds from the ring, so is partially delocalised into the pi system, increasing the electron density of the ring, increasing its ability to attract electrophiles
What happens if you shake phenol with orange, bromine water?
It will react, decolorizing the orange colour, smelling of antiseptic
What are amines?
Molecules with the functional group NR2, where R is a hydrgogen or an alkyl group
How can you make an aliphatic amine?
Heat excess ethanolic ammonia with a halogenoalkane, which will result in a mixture of primary, secondary and tertiary amines
When you reduce a nitrile using lithium aluminium hydride, followed by dilute HCl what do you form?
A primary amine, but as lithium aluminium hydride is very expensive, there are other ways…
What is the other way to form a primary amine from a nitrile?
React with hydrogen using a nickel catalyst and a high temperature and pressure
How do you form an aromatic amine?
React a nitrobenzene with tin, conc HCl and reflux which forms a salt, then add NaOH to get the aromatic amine
What do bases do?
They accept protons
What does the strength of the base depend on?
How available the lone pair of electrons is on the nitrogen atom
What makes a lone pair of electrons more availble?
When the electron density is higher
Why are aromatic amines a relatively weak base?
The benzene ring pulls electrons to itself, which partially delocalises the nitrogen lone pair, decreasing the electron density of the lone pair, making it much less available and less able to attract protons
Why are primary aliphatic amines very strong bases?
The alkyl group pushes electrons onto the attached groups so the electron density on the nitrogen atom increases, making the lone pair much more available and able to attract protons
Rank , ammonia, aliphatic amines and aromatic amines in order of strength of base?
1-Aliphatic amines
2-Ammonia
3-Aromatic amines
Why are small amines soluble in water?
The amine group can form hydrogen bonds with the water molecules, forming alkaline solutions`
Why are larger amines less soluble in water?
They have stronger London Forces holding them together, so larger forces are required to overcome the London forces. The larger chains also disrupt the hydrogen bonding in water
What happens when you add a small amount of butylamine to copper(2) sulfate solution?
A pale blue precipitate is formed, followed by a deep blue solution when added in excess. The butylamine molecules surround the copper atom
How do you form a secondary and tertiary amine from a primary amine?
React with a halogenoalkane in a nucleophilic substitution reaction
How do you form an N-substituted amide form an acyl chloride?R
React with a primary amine at room temperature
What is the functional group of an amide?
C0NH2
What is an N-substituted amide?
An amide but the nitrogen atom is attached to another alkyl group instead of a hydrogen
How do you name a primary amide?
Use the stem of the carbon chain, followed by -amide.
How do you name an N-substituted amide?
N - alkyl(from nitrogen) , stem of carbon chain -amide
How do you make a primary amide from an acyl chloride?
React with ammonia at room temp
How do you form an N-substituted amide from an acyl chloride?
React with a primary amine at room temperature
What does condensation polymerisation involve?
Two types of monomers, each monomer having at least 2 functional groups creating polymer chains
Why is it called ‘condensation’ polymerisation?
Each time a link is formed, a small molecule (often water) is lost
What is formed when dicarboxylic acids react with diamines?
Polyamides, and a water molecule is eliminated
What are amino acids?
Molecules which contain both an amine and a carboxylic group
When you react amino acids together, what do you form?
Proteins
How can you identify the amino acid monomers in a protein?
Use chromotography
What happens when you react dicarboxylic acids and diols?
You form polyesters
How do you find the monomers of a condensation polymer?
1-Find the amide or ester link
2-Add an H or OH to both ends of both molecules
How do you find a condensation polymer from monomer groups?
Draw the monomer molecules next to each other and join the functional groups to find the amide or ester link
What 2 groups make up and amino acid?
NH2 and COOH
Amino acids are amphoteric, what does this mean?
They can act as both a base and an acid as they have both and NH2 group and a COOH group
Amino acids exist as a zwitterion, what is a zwitterion?
An overall neutral molecule with a + and - charge in different parts of the molecule
In acidic conditions (low pH), what happens to the NH2 group of the amino acid?
It is protonated (+charge)
In basic conditions (high pH), what happens to the COOH group of the amino acid?
Negatively charged and is likely to lose its proton
Why are most amino acids chiral?
The carbon atom is a chiral centre
As most amino acids are chiral, what happens when plane-prolarised light is passed through an aqeuous solution containing 1 enantiomer of an amino acid?
It gets rotated
In paper chromatography, what is the Rf value?
Rf = distance travelled by spot / distance travelled by solvent
Describe a paper chromatography experiment to identify different amino acids:
1-Draw line at bottom of paper, place in a beaker and fill solvent to just below the line
2-Mark the point where the solvent reached (solvent front)
3-Identify spots of different chemicals on paper, and compare their Rf values and colours to find the amino acid etc,
