What are halogenoalkanes ?

Halogenoalkanes are organic compounds in which one or more hydrogen atoms in an alkane have been replaced by a halogen atom (F, Cl, Br or I). They are classified as primary, secondary or tertiary depending on the carbon bearing the halogen.

What prefixes are used when naming halogenoalkanes to indicate each halogen?

Fluorine — fluoro Chlorine — chloro Bromine — bromo Iodine — iodo

Define primary halogenoalkane

A primary halogenoalkane is one in which the halogen is attached to a carbon that is bonded to only one other alkyl group. For example, 1-bromopropane (CH 3 CH 2 CH 2 Br) is a primary halogenoalkane.

Define secondary halogenoalkane

A secondary halogenoalkane is one in which the halogen is attached to a carbon that is bonded to two other alkyl groups. For example, 2-bromopropane (CH 3 CH(Br)CH 3 ) is a secondary halogenoalkane.

Define tertiary halogenoalkane

A tertiary halogenoalkane is one in which the halogen is attached to a carbon that is bonded to three other alkyl groups.

The compound CH 3 CH(Cl)CH 2 Br is named .......... -bromo- .......... -chloropropane because substituents are listed .......... .

The compound CH 3 CH(Cl)CH 2 Br is named 1 -bromo- 2 -chloropropane because substituents are listed alphabetically .

True or False? When naming a halogenoalkane with two different halogens, the substituents are listed in order of increasing atomic number.

False. Substituents in halogenoalkane names are listed in alphabetical order, not by atomic number. For example, bromo is listed before chloro even though bromine has a higher atomic number.

How do you determine whether a halogenoalkane is primary , secondary or tertiary ?

Count the number of alkyl groups attached to the carbon bearing the halogen. One alkyl group = primary. Two alkyl groups = secondary. Three alkyl groups = tertiary.

What nucleophile and conditions are used to convert a halogenoalkane into an alcohol ?

The nucleophile is the hydroxide ion (OH - ). The halogenoalkane is warmed with an aqueous solution of NaOH or KOH, using a small amount of ethanol as a co-solvent.

Why do aqueous NaOH and ethanolic NaOH give different products when reacting with a halogenoalkane?

Aqueous NaOH causes nucleophilic substitution , producing an alcohol. Ethanolic NaOH causes elimination , producing an alkene because the solvent favours the elimination pathway.

When bromoethane reacts with ethanolic KCN under reflux, the product is .......... and the carbon chain length .......... by one.

When bromoethane reacts with ethanolic KCN under reflux, the product is propanenitrile and the carbon chain length increases by one.

What is the product called when a halogenoalkane reacts with KCN?

The product is a nitrile . The cyanide ion (CN - ) acts as the nucleophile and replaces the halogen, extending the carbon chain by one carbon atom.

What are the conditions for forming a primary amine from a halogenoalkane?

The halogenoalkane is heated under pressure with an excess of ethanolic ammonia (NH 3 dissolved in ethanol). Excess ammonia is used to prevent further substitution of the amine product.

True or False? Fluoroalkanes react the fastest in nucleophilic substitution reactions with aqueous NaOH.

False. Fluoroalkanes do not react at all because the C-F bond has the highest bond enthalpy. Iodoalkanes react the fastest because the C-I bond is the weakest.

Which halogenoalkane reacts fastest with aqueous NaOH in nucleophilic substitution?

Fluoroalkanes do not react at all because the C-F bond has the highest bond enthalpy. Iodoalkanes react the fastest because the C-I bond is the weakest.

Why is OH - a better nucleophile than water when reacting with halogenoalkanes?

OH - carries a full negative charge , making it more electron-rich and a stronger nucleophile. In water, the oxygen only carries a partial negative charge (δ-), so the reaction is much slower.

What organic product forms when a halogenoalkane is heated with ethanolic sodium hydroxide ?

An alkene is formed via an elimination reaction. The C-X bond breaks heterolytically, expelling a halide ion (X - ) and forming a C=C double bond.

Define nucleophilic substitution

A nucleophilic substitution is a reaction in which a nucleophile attacks a carbon atom carrying a partial positive charge and replaces an atom or group carrying a partial negative charge.

Why do halogenoalkanes undergo nucleophilic substitution reactions?

The C-X bond is polar because of the large difference in electronegativity between carbon and the halogen. This gives the carbon atom a partial positive charge (δ+), making it susceptible to attack by a nucleophile.

Describe the mechanism of nucleophilic substitution when bromoethane reacts with aqueous OH - .

The OH - ion donates a lone pair to the δ+ carbon of the C-Br bond, forming a new C-O bond. Simultaneously, the C-Br bond breaks heterolytically and the bromide ion (Br - ) is expelled as a leaving group, producing ethanol.

In the C-X bond of a halogenoalkane, the carbon carries a partial .......... charge and the halogen carries a partial .......... charge, because halogens are more .......... than carbon.

In the C-X bond of a halogenoalkane, the carbon carries a partial positive charge and the halogen carries a partial negative charge, because halogens are more electronegative than carbon.

Describe the two steps in the reaction of chloromethane with excess ammonia to form methylamine.

Step 1: NH 3 attacks the δ+ carbon of CH 3 Cl by nucleophilic substitution, forming [CH 3 NH 3 ] + Cl - . Step 2: A second NH 3 molecule acts as a base, removing a proton from [CH 3 NH 3 ] + to give methylamine (CH 3 NH 2 ) and NH 4 + Cl - .

True or False? In nucleophilic substitution, the nucleophile attacks the carbon atom that carries a partial negative charge.

False. The nucleophile attacks the carbon atom that carries a partial positive charge (δ+). The atom carrying the partial negative charge is the one that is replaced (e.g. the halogen).

What does a curly arrow represent in a nucleophilic substitution mechanism?

A curly arrow shows the movement of an electron pair . A double-headed curly arrow is drawn from the lone pair of the nucleophile to the δ+ carbon, and from the C-X bond to the leaving halogen atom.

Why must excess ammonia be used when forming a primary amine from a halogenoalkane?

The primary amine product is itself a nucleophile and can undergo further substitution reactions with the halogenoalkane. Using excess ammonia favours the formation of the primary amine over further substituted products.

Describe the experimental method used to compare the rate of hydrolysis of different halogenoalkanes.

Three test tubes are set up in a 50°C water bath, each containing ethanol and acidified silver nitrate solution. A few drops of a chloroalkane, bromoalkane and iodoalkane are added to separate test tubes and a stopwatch is started. The time for each precipitate to form is recorded.

Why is no precipitate formed when a fluoroalkane is used in the silver nitrate hydrolysis test?

Silver fluoride is soluble , so no precipitate forms even if fluoride ions are produced. This also confirms that fluoroalkanes are the least reactive halogenoalkanes and do not undergo significant hydrolysis.

True or False? Chloroalkanes hydrolyse the fastest because the C-Cl bond is the weakest C-X bond.

False. Chloroalkanes hydrolyse the slowest among reactive halogenoalkanes. The C-Cl bond has the highest bond enthalpy , making it the hardest to break.

State the order of reactivity of halogenoalkanes in hydrolysis, from fastest to slowest .

Iodoalkanes > bromoalkanes > chloroalkanes. Fluoroalkanes do not react at all. The weaker the C-X bond (lower bond enthalpy), the faster the halide ion is released and the faster the hydrolysis.

Why does the C-I bond break most easily in nucleophilic substitution?

The C-I bond has the lowest bond enthalpy of all C-X bonds, so it requires the least energy to break. This means iodide ions are released fastest, producing the yellow silver iodide precipitate most quickly in the silver nitrate test.

In the silver nitrate hydrolysis test, the .......... precipitate forms fastest and the .......... precipitate forms slowest, showing that .......... alkanes are the most reactive.

In the silver nitrate hydrolysis test, the yellow precipitate forms fastest and the white precipitate forms slowest, showing that iodo alkanes are the most reactive.

In a hydrolysis comparison between 1-chlorobutane, 2-chlorobutane and 2-chloro-2-methylpropane, which reacts the fastest and which the slowest ?

2-chloro-2-methylpropane (the tertiary halogenoalkane) reacts the fastest. 1-chlorobutane (the primary halogenoalkane) reacts the slowest.

Why is ethanol included in the silver nitrate hydrolysis test mixture?

Ethanol acts as a co-solvent to help dissolve the halogenoalkane, which is not miscible with water alone. This ensures the halogenoalkane mixes with the aqueous silver nitrate so the hydrolysis reaction can occur.

Why is the C-X bond in a halogenoalkane polar?

There is a large difference in electronegativity between the halogen and the carbon atom. The halogen is more electronegative, giving it a δ- charge and leaving the carbon with a δ+ charge.

Bond enthalpy is the energy required to break one mole of a covalent bond in the gaseous state, averaged across similar chemical environments. A higher bond enthalpy means the bond is stronger and harder to break.

State the order of C-X bond enthalpies from weakest to strongest across the halogenoalkanes.

C-I < C-Br < C-Cl < C-F. The C-I bond is the weakest and easiest to break, making iodoalkanes the most reactive. The C-F bond is the strongest, making fluoroalkanes the least reactive.

Why do different halogenoalkanes have different rates of substitution reactions?

The rate depends on the bond enthalpy of the C-X bond. Weaker C-X bonds (lower bond enthalpy) break more easily, allowing the nucleophile to replace the halogen faster. The C-I bond is weakest, so iodoalkanes react fastest.

True or False? Based on bond polarity alone, fluoroalkanes would be predicted to be the most reactive halogenoalkanes.

True. Electronegativity decreases down Group 17, so the C-F bond is the most polar, which would suggest it is most reactive. However, bond enthalpy is a greater factor, and the C-F bond is the strongest, so fluoroalkanes are actually the least reactive.

Why are fluoroalkanes the least reactive halogenoalkanes despite having the most polar C-X bond?

The C-F bond has the highest bond enthalpy of all C-X bonds, making it very difficult to break. Bond enthalpy is a greater factor than bond polarity in determining reactivity, so fluoroalkanes rarely undergo substitution reactions.

In substitution reactions of halogenoalkanes, the C-X bond undergoes .......... fission. The .......... alkane reacts fastest because its C-X bond has the .......... bond enthalpy.

In substitution reactions of halogenoalkanes, the C-X bond undergoes heterolytic fission. The iodo alkane reacts fastest because its C-X bond has the lowest bond enthalpy.

In the reaction CH 3 CH 2 I + OH - → CH 3 CH 2 OH + I - , why does the C-I bond break rather than a C-H bond?

The C-I bond is polar (δ+ on carbon, δ- on iodine), making the carbon susceptible to nucleophilic attack. The C-I bond also has a lower bond enthalpy than C-H bonds, so it is easier to break during the substitution reaction.

International A Level (IAL)ChemistryEdexcelFlashcards2. Energetics, Group Chemistry, Halogenoalkanes & AlcoholsOrganic Chemistry: Halogenoalkanes

Organic Chemistry: Halogenoalkanes (Edexcel International A Level (IAL) Chemistry): Flashcards

Exam code: YCH11

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FrontHalogenoalkanes - Introduction
What are halogenoalkanes?
FrontHalogenoalkanes - Introduction
What prefixes are used when naming halogenoalkanes to indicate each halogen?
BackHalogenoalkanes - Introduction
Fluorine — fluoro
Chlorine — chloro
Bromine — bromo
Iodine — iodo

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Cards in this collection (41)

What are halogenoalkanes?
Halogenoalkanes are organic compounds in which one or more hydrogen atoms in an alkane have been replaced by a halogen atom (F, Cl, Br or I).
They are classified as primary, secondary or tertiary depending on the carbon bearing the halogen.
What prefixes are used when naming halogenoalkanes to indicate each halogen?
1. Fluorine — fluoro
2. Chlorine — chloro
3. Bromine — bromo
4. Iodine — iodo
Define primary halogenoalkane
A primary halogenoalkane is one in which the halogen is attached to a carbon that is bonded to only one other alkyl group.
For example, 1-bromopropane (CH₃CH₂CH₂Br) is a primary halogenoalkane.
Define secondary halogenoalkane
A secondary halogenoalkane is one in which the halogen is attached to a carbon that is bonded to two other alkyl groups.
For example, 2-bromopropane (CH₃CH(Br)CH₃) is a secondary halogenoalkane.
Define tertiary halogenoalkane
A tertiary halogenoalkane is one in which the halogen is attached to a carbon that is bonded to three other alkyl groups.
The compound CH₃CH(Cl)CH₂Br is named .......... -bromo- .......... -chloropropane because substituents are listed .......... .
The compound CH₃CH(Cl)CH₂Br is named 1-bromo-2-chloropropane because substituents are listed alphabetically.
True or False?
When naming a halogenoalkane with two different halogens, the substituents are listed in order of increasing atomic number.
False.
Substituents in halogenoalkane names are listed in alphabetical order, not by atomic number. For example, bromo is listed before chloro even though bromine has a higher atomic number.
How do you determine whether a halogenoalkane is primary, secondary or tertiary?
Count the number of alkyl groups attached to the carbon bearing the halogen.
One alkyl group = primary. Two alkyl groups = secondary. Three alkyl groups = tertiary.
What nucleophile and conditions are used to convert a halogenoalkane into an alcohol?
The nucleophile is the hydroxide ion (OH^-).
The halogenoalkane is warmed with an aqueous solution of NaOH or KOH, using a small amount of ethanol as a co-solvent.
Why do aqueous NaOH and ethanolic NaOH give different products when reacting with a halogenoalkane?
Aqueous NaOH causes nucleophilic substitution, producing an alcohol.
Ethanolic NaOH causes elimination, producing an alkene because the solvent favours the elimination pathway.
When bromoethane reacts with ethanolic KCN under reflux, the product is .......... and the carbon chain length .......... by one.
When bromoethane reacts with ethanolic KCN under reflux, the product is propanenitrile and the carbon chain length increases by one.
What is the product called when a halogenoalkane reacts with KCN?
The product is a nitrile.
The cyanide ion (CN^-) acts as the nucleophile and replaces the halogen, extending the carbon chain by one carbon atom.
What are the conditions for forming a primary amine from a halogenoalkane?
The halogenoalkane is heated under pressure with an excess of ethanolic ammonia (NH₃ dissolved in ethanol).
Excess ammonia is used to prevent further substitution of the amine product.
True or False?
Fluoroalkanes react the fastest in nucleophilic substitution reactions with aqueous NaOH.
False.
Fluoroalkanes do not react at all because the C-F bond has the highest bond enthalpy. Iodoalkanes react the fastest because the C-I bond is the weakest.
Which halogenoalkane reacts fastest with aqueous NaOH in nucleophilic substitution?
Fluoroalkanes do not react at all because the C-F bond has the highest bond enthalpy. Iodoalkanes react the fastest because the C-I bond is the weakest.
Why is OH^- a better nucleophile than water when reacting with halogenoalkanes?
OH^- carries a full negative charge, making it more electron-rich and a stronger nucleophile.
In water, the oxygen only carries a partial negative charge (δ-), so the reaction is much slower.
What organic product forms when a halogenoalkane is heated with ethanolic sodium hydroxide?
An alkene is formed via an elimination reaction.
The C-X bond breaks heterolytically, expelling a halide ion (X^-) and forming a C=C double bond.
Define nucleophilic substitution
A nucleophilic substitution is a reaction in which a nucleophile attacks a carbon atom carrying a partial positive charge and replaces an atom or group carrying a partial negative charge.
Why do halogenoalkanes undergo nucleophilic substitution reactions?
The C-X bond is polar because of the large difference in electronegativity between carbon and the halogen.
This gives the carbon atom a partial positive charge (δ+), making it susceptible to attack by a nucleophile.
Describe the mechanism of nucleophilic substitution when bromoethane reacts with aqueous OH^-.
The OH^- ion donates a lone pair to the δ+ carbon of the C-Br bond, forming a new C-O bond.
Simultaneously, the C-Br bond breaks heterolytically and the bromide ion (Br^-) is expelled as a leaving group, producing ethanol.
In the C-X bond of a halogenoalkane, the carbon carries a partial .......... charge and the halogen carries a partial .......... charge, because halogens are more .......... than carbon.
In the C-X bond of a halogenoalkane, the carbon carries a partial positive charge and the halogen carries a partial negative charge, because halogens are more electronegative than carbon.
Describe the two steps in the reaction of chloromethane with excess ammonia to form methylamine.
Step 1: NH₃ attacks the δ+ carbon of CH₃Cl by nucleophilic substitution, forming [CH₃NH₃]⁺Cl^-.
Step 2: A second NH₃ molecule acts as a base, removing a proton from [CH₃NH₃]⁺ to give methylamine (CH₃NH₂) and NH₄⁺Cl^-.
True or False?
In nucleophilic substitution, the nucleophile attacks the carbon atom that carries a partial negative charge.
False.
The nucleophile attacks the carbon atom that carries a partial positive charge (δ+). The atom carrying the partial negative charge is the one that is replaced (e.g. the halogen).
What does a curly arrow represent in a nucleophilic substitution mechanism?
A curly arrow shows the movement of an electron pair.
A double-headed curly arrow is drawn from the lone pair of the nucleophile to the δ+ carbon, and from the C-X bond to the leaving halogen atom.
Why must excess ammonia be used when forming a primary amine from a halogenoalkane?
The primary amine product is itself a nucleophile and can undergo further substitution reactions with the halogenoalkane.
Using excess ammonia favours the formation of the primary amine over further substituted products.
Describe the experimental method used to compare the rate of hydrolysis of different halogenoalkanes.
Three test tubes are set up in a 50°C water bath, each containing ethanol and acidified silver nitrate solution.
A few drops of a chloroalkane, bromoalkane and iodoalkane are added to separate test tubes and a stopwatch is started. The time for each precipitate to form is recorded.
Why is no precipitate formed when a fluoroalkane is used in the silver nitrate hydrolysis test?
Silver fluoride is soluble, so no precipitate forms even if fluoride ions are produced.
This also confirms that fluoroalkanes are the least reactive halogenoalkanes and do not undergo significant hydrolysis.
True or False?
Chloroalkanes hydrolyse the fastest because the C-Cl bond is the weakest C-X bond.
False.
Chloroalkanes hydrolyse the slowest among reactive halogenoalkanes. The C-Cl bond has the highest bond enthalpy, making it the hardest to break.
State the order of reactivity of halogenoalkanes in hydrolysis, from fastest to slowest.
Iodoalkanes > bromoalkanes > chloroalkanes.
Fluoroalkanes do not react at all. The weaker the C-X bond (lower bond enthalpy), the faster the halide ion is released and the faster the hydrolysis.
Why does the C-I bond break most easily in nucleophilic substitution?
The C-I bond has the lowest bond enthalpy of all C-X bonds, so it requires the least energy to break.
This means iodide ions are released fastest, producing the yellow silver iodide precipitate most quickly in the silver nitrate test.
In the silver nitrate hydrolysis test, the .......... precipitate forms fastest and the .......... precipitate forms slowest, showing that .......... alkanes are the most reactive.
In the silver nitrate hydrolysis test, the yellow precipitate forms fastest and the white precipitate forms slowest, showing that iodo alkanes are the most reactive.
In a hydrolysis comparison between 1-chlorobutane, 2-chlorobutane and 2-chloro-2-methylpropane, which reacts the fastest and which the slowest?
2-chloro-2-methylpropane (the tertiary halogenoalkane) reacts the fastest.
1-chlorobutane (the primary halogenoalkane) reacts the slowest.
Why is ethanol included in the silver nitrate hydrolysis test mixture?
Ethanol acts as a co-solvent to help dissolve the halogenoalkane, which is not miscible with water alone.
This ensures the halogenoalkane mixes with the aqueous silver nitrate so the hydrolysis reaction can occur.
Why is the C-X bond in a halogenoalkane polar?
There is a large difference in electronegativity between the halogen and the carbon atom.
The halogen is more electronegative, giving it a δ- charge and leaving the carbon with a δ+ charge.
Define bond enthalpy
Bond enthalpy is the energy required to break one mole of a covalent bond in the gaseous state, averaged across similar chemical environments.
A higher bond enthalpy means the bond is stronger and harder to break.
State the order of C-X bond enthalpies from weakest to strongest across the halogenoalkanes.
C-I < C-Br < C-Cl < C-F.
The C-I bond is the weakest and easiest to break, making iodoalkanes the most reactive. The C-F bond is the strongest, making fluoroalkanes the least reactive.
Why do different halogenoalkanes have different rates of substitution reactions?
The rate depends on the bond enthalpy of the C-X bond.
Weaker C-X bonds (lower bond enthalpy) break more easily, allowing the nucleophile to replace the halogen faster. The C-I bond is weakest, so iodoalkanes react fastest.
True or False?
Based on bond polarity alone, fluoroalkanes would be predicted to be the most reactive halogenoalkanes.
True.
Electronegativity decreases down Group 17, so the C-F bond is the most polar, which would suggest it is most reactive. However, bond enthalpy is a greater factor, and the C-F bond is the strongest, so fluoroalkanes are actually the least reactive.
Why are fluoroalkanes the least reactive halogenoalkanes despite having the most polar C-X bond?
The C-F bond has the highest bond enthalpy of all C-X bonds, making it very difficult to break.
Bond enthalpy is a greater factor than bond polarity in determining reactivity, so fluoroalkanes rarely undergo substitution reactions.
In substitution reactions of halogenoalkanes, the C-X bond undergoes .......... fission. The .......... alkane reacts fastest because its C-X bond has the .......... bond enthalpy.
In substitution reactions of halogenoalkanes, the C-X bond undergoes heterolytic fission. The iodo alkane reacts fastest because its C-X bond has the lowest bond enthalpy.
In the reaction CH₃CH₂I + OH^- → CH₃CH₂OH + I^-, why does the C-I bond break rather than a C-H bond?
The C-I bond is polar (δ+ on carbon, δ- on iodine), making the carbon susceptible to nucleophilic attack.
The C-I bond also has a lower bond enthalpy than C-H bonds, so it is easier to break during the substitution reaction.

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