Benzene Flashcards
What are aromatics?
contain a benzene ring
What are aromatics used in?
dyes, explosives, drugs
What is the empirical formula and Mr of benzene?
CH, 78
Compared to alkenes, how reactive is benzene?
fairly unreactive
What reaction indicates that benzene is a cyclic compound that contains 3 C=C?
when benzene is hydrogenated, 3 moles of H2 gas react with each mole of benzene to form cyclohexane
What did Kekulé suggest about benzene?
that is has a cyclic structure with alternating single and double C=C bonds
What is the problem with Kekulé stuctures?
-if benzene is unsaturated, it should undergo electrophilic addition reactions readily but it does not
-the reaction of benzene with chlorine is expected to form 2 different products, but only 1 product is formed. this indicates that all H atoms in benzene are equivalent
-x-ray diffraction studies have shown that benzene is planar and a regular hexagon, so all C-C bonds are equal, in kekulé’s structure it would have different bond lengths for C-C and C=C
-the enthalpy of hydrogenation values
What is the bond length for C-C, C=C, and bonding in benzene?
c-c — 0.154nm
c=c — 0..134nm
benzene — 0.139nm
What do the enthalpies of hydrogenation show for benzene?
that benzene is much more stable than kekulé’s structure, as when cyclohexene is hydrogenated 120 kJ/mol-1 is released, so we can assume that the enthalpy of hydrogenation for kekulé’s structure is three times this (360kJ/mol-1)
-however, the observed value for benzene is only 208 kJ/mol-1
What does the fact that benzenes enthalpy of hydrogenation is lower than kekule’s structure mean?
that it requires less energy to break the double bonds in benzene than 3 double bonds in an alkene
Why is benzene more stable than expected?
due to delocalisation stabilisation- the pi electrons in the C=C are delocalised around the ring, and the delocalisation brings extra stability
Explain the structure of benzene?
-6 carbon atoms are planar and arranged in a regular hexagon, (all bond angles are 120), so each carbon atom is bonded to 2 other carbons and 1 hydrogen, leaving it with 1 unused pi electron. the 6 of these produce electron density above and below the plane of the ring
What is resonance/delocalisation energy?
the increase in stability associated with electron delocalisation
What do you need to include in an electrophilic substitution reaction?
step 1- formation of electrophile
step 2- mechanism
What is the equation for making nitronium ion?
HNO3 + 2H2SO4 –> +NO2 + H3O+ +2HSO4-
What is the structure of a nitronium ion?
O=N+=O
What are the conditions for the nitration of benzene and why?
conc H2SO4 AND conc HNO3, and 50C, as above this multiple substitutions can occur
Why is nitration important?
nitrated benzene derivatives used as explosives- the methyl group is electron releasing so that increasing the attraction to electrophiles even more
What are the two ways that nitro groups can be reduced, and are they specific or not?
-catalytic hydrogenation, H2/Ni (non-specific)
-metal /acid combos (eg Sn/conc HCl), specfic
What are primary aromatic amines used to make?
azo dyes
How do you name aromatics with only 1 substituent?
just add to front- eg bromobenzene
How do you name aromatics with more than one substituent?
-lowest possible number
-listed alphabetically
What happens to alkyl side chains?
given number 1
How is polystyrene made?
addition polymerisation of the monomer phenylethene
What happens in a Friedel-Crafts Acylation?
-an acyl group is joined onto the benzene ring to form a ketone, leads to C-C bond formation, and the catalyst is an acylium cation (R-C+=O)
How are aromatic ketones made?
benzene reacts with acyl chlorides/acid anhydrides in the presence of an aluminium chloride catalyst (product HCl)
What is the equation for making the aluminium chloride electrophile?
CH3COCl + AlCl3 –> CH3-C+=O + AlCl4-
What does the AlCl3 catalyst act as?
a lewis acid (electron pair acceptors)
Why does the acylation of benzene not occur by nucleophilic addition?
electron pairs in nucleophiles repelled by the delocalised electrons in benzene
