Chapter 25 Flashcards by Shivani Pankhania

What is the molecular formula of benzene?

C₆H₆

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What colour is benzene?

Colourlesss

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What type of smell does benzene have?

Sweet

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How flammable is benzene?

Highly flammable

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What is a carcinogen?

Can cause cancer

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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

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What is benzene classed as?

1) aromatic hydrocarbon

2) arene

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What is the Kekulé model?

1) six membered ring of carbon atoms

2) joined by alternate single and double bonds

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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

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What do alkenes decolourise?

Bromine water

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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

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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

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What is hydrogenation?

The addition of hydrogen to an unsaturated compound

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What is the IUPAC name of the Kekulé model?

cyclohexa-1,3,5-triene

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When cyclohexene is hydrogenated, how many double bonds react with hydrogen?

One

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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

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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

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What groups are prefixes to benzene?

1) short alkyl chains
2) halogens
3) nitro groups

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What are aromatic compounds with one substituent group?

Monsubstituted

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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

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When is the prefix phenyl used instead of benzene?

When benzene is considered to be a substituent

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What are the three exceptions when naming aromatic compounds?

1) Benzoic acid (benzenecarboxylic acid)
2) phenylamine
3) benzaldehyde (benzenecarbaldehyde)

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How many substituent groups do disubstituted compounds have?

Two

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How are substituent groups listed?

1) alphabetical order

2) using the smallest numbers possible

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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+