MC test: Halogenoalkanes & benzene

Multiple choice test on 10.2(3) Halogenoalkanes & benzene

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

Which factors help to explain why halogenoalkanes are generally more reactive than alkanes?

I. The molecules of halogenoalkanes are more polar than molecules of alkanes.

II. All C−halogen bonds are weaker than C−C and C−H bonds.

III. All the halogens are more electronegative than hydrogen and carbon.

I and III only

II and III only

I, II and III

I and II only

It is tempting to think that all three factors are correct. The C−Cl, C−Br and C−I bonds are indeed weaker than the C−C and C−H bonds. However the C−F bond is a very strong bond (492 kJ mol⁻¹) and is much stronger than the C−C (346 kJ mol⁻¹) and C−H (414 kJ mol⁻¹) bonds.

Methane (boiling point (− 161 ^oC) and tetrafluoromethane (boiling point − 128 ^oC) are both greenhouse gases. The C−H bond enthalpy is 414 kJ mol⁻¹and the C−F bond enthalpy is 492 kJ mol⁻¹.

Which statements about methane and tetrafluoromethane are correct?

I. The molecules of both methane and tetrafluoromethane have a tetrahedral structure.

II. Methane is more reactive chemically than tetrafluoromethane.

III. Tetrafluoromethane is considerably more polar than methane.

I, II and III

II and III only

I and III only

I and II only

CH₄ and CF₄ will both have a regular tetrahedral shape. Although individual C−F bonds are polar due to the high electronegativity of fluorine, CF₄ is a non-polar molecule as the resultant dipole will be zero. Methane combusts in air whereas tetrafluoromethane does not burn due to the high C−F bond enthalpy. Note than unlike Freons, such as CF₂Cl₂, tetrafluoromethane does not destroy ozone in the ozone layer as the C−F bond is too strong to be broken homolytically by ultraviolet radiation.

Which reagents can react together to give 1-chlorobutane as a product ?

but-2-ene and hydrogen chloride

butane and chlorine in ultraviolet light

but-1-ene and chlorine

butane and hydrochloric acid

Butane does not react with hydrochloric acid. But-2-ene with hydrogen chloride will produce 2-chlorobutane. But-1-ene with chlorine will produce 1,2-dichlorobutane. The reaction between butane with chlorine in ultraviolet light proceeds by a free radical mechanism and the substitution may occur on any of the carbon atoms so 1-chlorobutane will be among the products.

Which reactions produce butan-2-ol?

I. warming 2-iodobutane with sodium hydroxide solution

II. shining ultraviolet light on a mixture of butane and water

III. heating but-2-ene with steam in the presence of an acid catalyst

I and III only

I and II only

II and III only

I, II and III

Butane does not react with water in ultraviolet light as ultraviolet light does not break the O−H bonds in water or the C−H bonds in water. Steam can add across the double bond in but-2-ene and nucleophilic substitution occurs when 2-iodobutane reacts with hydroxide ions.

Which statement explains why halogenoalkanes react with nucleophiles?

The bond between carbon and the halogen in all halogenoalkanes is ionic so nucleophiles are attracted to the positive carbon ion.

The small atomic radius of a carbon atom enables the six protons in its nucleus to attract nucleophiles.

The carbon atom bonded to the halogen atom has a vacant orbital which can accept a pair of electrons.

Because halogens are more electronegative than carbon they attract the bonding pair of electrons more strongly making the carbon atom electron deficient with a small positive charge, δ⁺.

The carbon atom is bonded covalently to the halogen atom in halogenoalkanes but because of the greater electronegativity of the halogen atom the bond is polar and nucleophiles, such as the hydroxide ion, are attracted to the electron deficient carbon atom.

Which is a secondary halogenoalkane?

2-iodobutane

1-chloropropane

2-chloro-2-methylpropane

1,2-dibromoethane

1,2-dibromoethane and 1-chloropropane are primary halogenoalkanes as there is only one R− group attached to the carbon atom bonded to the halogen atom. 2-chloro-2-methylpropane is tertiary as there are three R− groups bonded to the carbon atom bonded to the chlorine atom.

What will be the organic product from the reaction between warm dilute aqueous sodium hydroxide solution and 1-bromobutane?

but-2-ene

but-1-ene

butan-1-ol

butan-2-ol

This is an example of a nucleophilic substitution reaction. CH₃CH₂CH₂CH₂Br(aq) + NaOH(aq) → CH₃CH₂CH₂CH₂OH(aq) + NaBr(aq)

Which reagent cannot act as a nucleophile?

the hydroxide ion, OH⁻

water, H₂O

methane, CH₄

ammonia, NH₃

Nucleophiles must possess at least one non-bonding pair of electrons.

Benzene, C₆H₆ can react with hydrogen at 180 ^oC in the presence of a nickel catalyst to form cyclohexane, C₆H₁₂.
Which terms can be used to describe what happens to benzene in this reaction?

I. reduction

II. hydrogenation

III. addition

I and II only

II and III only

I, II and III

I and III only

Reduction and hydrogenation both involve the addition of hydrogen.

Benzene can react with chlorine in the presence of aluminium chloride to form chlorobenzene.

Which terms can be used to describe what happens to benzene in this reaction?

I. reduction

II. substitution

III. chlorination

II and III only

I, II and III

I and III only

I and II only

The removal of hydrogen is one of the definitions of oxidation, so benzene is oxidized when it undergoes a substitution reaction with chlorine.

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MC test: Halogenoalkanes & benzene

Multiple choice test on 10.2(3) Halogenoalkanes & benzene

MC test: Halogenoalkanes & benzene