Chapter 25 Flashcards
What is the molecular formula of benzene?
C₆H₆
What colour is benzene?
Colourlesss
What type of smell does benzene have?
Sweet
How flammable is benzene?
Highly flammable
What is a carcinogen?
Can cause cancer
What is the structure of benzene?
1) hexagonal ring of six carbon atoms
2) carbon atoms joined to two other carbon atoms and to one hydrogen atom
What is benzene classed as?
1) aromatic hydrocarbon
2) arene
What is the Kekulé model?
1) six membered ring of carbon atoms
2) joined by alternate single and double bonds
What is the evidence to disprove Kekulé’s model?
1) the lack of reactivity of benzene
2) the lengths of the carbon-carbon bonds in benzene
3) hydrogenation enthalpies
What do alkenes decolourise?
Bromine water
How does the lack of reactivity of benzene disprove Kekulé’s model?
1) benzene does not undergo electrophilic addition reactions
2) benzene does not decolourise bromine under normal conditions
How do the lengths of the carbon-carbon bonds in benzene disprove Kekulé’s model?
All the bonds in benzene are 0.139nm in length, between the length of a single bond, 0.153nm, and a double bond, 0.134nm
What is hydrogenation?
The addition of hydrogen to an unsaturated compound
What is the IUPAC name of the Kekulé model?
cyclohexa-1,3,5-triene
When cyclohexene is hydrogenated, how many double bonds react with hydrogen?
One
What is the structure of the delocalised model of benzene?
1) planar
2) cyclic
3) hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms
What is the bonding in the delocalised model of benzene?
1) each carbon atom uses three of its available four electrons i bonding to two other carbon atoms
2) each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms
3) adjacent p-orbital electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density
4) overlapping of the p-orbitals creates a system of pi-bonds which spread over all six of the carbon atoms in the ring structure
5) six electrons occupying system of pi-bonds are delocalised
What groups are prefixes to benzene?
1) short alkyl chains
2) halogens
3) nitro groups
What are aromatic compounds with one substituent group?
Monsubstituted
When is benzene considered to be substituent?
When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms
When is the prefix phenyl used instead of benzene?
When benzene is considered to be a substituent
What are the three exceptions when naming aromatic compounds?
1) Benzoic acid (benzenecarboxylic acid)
2) phenylamine
3) benzaldehyde (benzenecarbaldehyde)
How many substituent groups do disubstituted compounds have?
Two
How are substituent groups listed?
1) alphabetical order
2) using the smallest numbers possible
What are the electrophilic substitution reactions of benzene?
1) nitration
2) halogenation
3) alkylation
4) acylation
What is nitrobenzene formed from?
Benzene + nitric acid
What are the conditions needed for the nitration of benzene?
1) sulfuric acid catalyst
2) 50°C
What piece of equipment is used to maintain a steady temperature?
Water bath
What could happen if the temperature of the nitration of benzene rises above 50°C?
Further substitution reactions may occur leading to the production of dinitrobenzene
What is an electrophile?
An atom (or group of atoms) which is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond
What is nitrobenzene an important starting material of?
1) dyes
2) pharmaceuticals
3) pesticides
What is the electrophile involved in the mechanism for the nitration of benzene?
The nitronium ion, NO₂+, produced by the reaction of concentrated nitric acid with concentrated sulfuric acid
What is the equation where concentrated nitric acid and concentrated sulfuric acid react together to produce nitronium ions?
HNO₃ + H₂SO₄ –> NO₂+ + HSO₄- + H₂O
What happens to the H+ ion formed from the nitration of benzene?
It reacts with the HSO₄- ion to regenerate the catalyst H₂SO₄
H+ + HSO₄- –> H₂SO₄
When do halogens react with benzene?
When a halogen carrier is present
Give some examples of common halogen carriers.
1) AlCl₃
2) AlBr₃
3) FeCl₃
4) FeBr₃
What are the conditions needed for the bromination of benzene?
1) room temperature and pressure
2) halogen carrier
What is the electrophile involved in the mechanism for the bromination of benzene?
Br+
What is the equation where bromine and a halogen carrier react together to produce bromonium ions?
Br₂ + FeBr₃ –> FeBr₄- + Br+
Br₂ + AlBr₃ –> AlBr₄- + Br+
What happens to the H+ ion formed from the bromination of benzene?
It reacts with the FeBr₄-/AlBr₄- ion to regenerate the halogen carrier catalyst
H+ + FeBr₄- –> FeBr₃ + HBr
H+ + AlBr₄- –> AlBr₃ + HBr
What is alkylation?
Can be used to add a carbon chain to the benzene ring
What is the alkylation of benzene?
The substitution of a hydrogen atom in the benzene ring by an alkyl group
How is the alkylation of benzene carried out?
By reacting benzene with a haloalkane in the presence of AlCl₃
What are alkylation reactions sometimes called?
Friedel-Crafts alkylation reactions after the two chemists who fist carried out the reaction
What is the acylation of benzene?
When benzene reacts with an acyl chloride in the presence of an AlCl₃ catalyst to form am aromatic ketone
What is the first member of the acyl chloride homologous series?
Ethanoyl chloride, CH₃COCl
What is produced when benzene and ethanoyl chloride react together?
Phenylethanone + hydrochloric acid
What do alkenes decolourise?
Bromine water by an electrophilic addition reaction
What is produced when cyclohexene and bromine react together?
1,2-dibromocyclohexane
Describe the electrophilic addition mechanism reaction between cyclohexene and bromine
1) π-bond in alkene contains delocalised e- above + below plane of 2 C-atoms in double bond. This produces an area of high electron density
2) localised e- in π-bond induce dipole in non-polar Br molecule making one Br atom slightly positive and the other slightly negative
3) slightly positive Br atom enables Br molecule to act like an electrophile
Why does benzene not react with bromine unless a halogen carrier catalyst is present?
1) benzene has delocalised π-electrons above + below the plane of C-atoms in the ring structure
2) the e- density around any two C-atoms in the benzene ring is less than that in a C=C double bond in an alkene
3) when a non-polar molecule approaches the benzene ring there is insufficient π-electron density around any two C-atoms to polarise the molecule, preventing any reaction taking place
What are phenols?
A type of organic chemical containing a hydroxyl, -OH, functional group directly bonded to an aromatic ring
What is the simplest member of the phenols?
Phenol, C₆H₅OH
What are compounds containing an -OH group bonded to a carbon side chain, rather than the aromatic ring, classified as?
Alcohols
Is phenol more or less soluble in water than alcohols?
Less soluble
Why is phenol less soluble in water than alcohols?
Due to the presence of the benzene ring
Is phenol a strong or weak acid?
Weak acid
Why is phenol classified as a weak acid?
When dissolved in water, phenol partially dissociates forming the phenoxide ion and a proton
Is phenol more or less acidic than alcohols?
More acidic
Is phenol more or less acidic than carboxylic acids?
Less acidic
What is Kₐ?
The acid dissociation constant
How can you tell the difference between a phenol and a carboxylic acid?
Carboxylic acids react with sodium carbonate to produce carbon dioxide
What does phenol react with sodium hydroxide to form?
Sodium phenoxide + water
What mechanism do phenols undergo?
Electrophilic substitution
What does phenol react with bromine water to form?
2,4,6-tribromophenol + 3HBr
What observation is made when bromine water is reacted with phenol?
1) bromine water is decolourised
2) white precipitate of 2,4,6-tribromophenol is formed
What is formed when phenol reacts with dilute nitric acid?
2-nitrophenol + H₂0
OR
4-nitrophenol + H₂O
Why do bromine and nitric acid react more readily with phenol than they do with benzene?
1) a lone pair of e- from the oxygen p-orbital of the -OH group is donated to the π-system of phenol
2) e- density of benzene ring in phenol is increased
3) increased e- density attracts electrophiles more strongly than with benzene
How would you carry out a test to distinguish between solutions of benzene and phenol?
Add bromine water, both would decolourise the bromine water but phenol would produce a white precipitate
Are 2- and 4- directing groups activating groups or deactivating groups?
Activating groups (except the halogens)
Are 3- directing groups activating groups or deactivating groups?
Deactivating groups
What groups are 2- and 4- directing?
1) -NH₂ or -NHR
2) -OH
3) -OR
4) -R or C₆H₅
5) -F, -Cl, -Br, -I
What groups are 3- directing?
1) RCOR
2) -COOR
3) SO₃H
4) -CHO
5) -COOH
6) -CN
7) -NO₂
8) -NR₃+
