What is homolytic fission ?

A polyester is a condensation polymer formed by the reaction between a diol and a dicarboxylic acid (or diacid chloride), with the repeat units linked by ester bonds (–COO–).

In heterolytic fission, the more .......... atom takes both electrons from the bond, forming a .......... ion and leaving behind a .......... ion.

In heterolytic fission, the more electronegative atom takes both electrons from the bond, forming a negative ion and leaving behind a positive ion.

What is a nucleophile ?

A nucleophile is an electron-rich species that can donate a pair of electrons to an electron-deficient species. Nucleophiles are attracted to positive charges or δ+ centres.

True or False? In organic reaction mechanisms, curly arrows represent the movement of single electrons.

False. Double-headed curly arrows represent the movement of electron pairs . Single-headed (fishhook) arrows represent single-electron movement in radical mechanisms.

A curly arrow in a mechanism begins at a .......... or a .......... pair of electrons, and points to the species that .......... the electrons.

A curly arrow in a mechanism begins at a bond or a lone pair of electrons, and points to the species that accepts the electrons.

Name the five main types of organic reaction.

Addition Substitution Elimination Hydrolysis Condensation

True or False? In organic chemistry, the symbol [O] in an equation represents one atom of oxygen from an oxidising agent.

True. [O] represents one atom of oxygen donated by an oxidising agent. When more than one oxygen atom is transferred, a coefficient is used. For example: RCH 2 OH + 2[O] → RCOOH + H 2 O The equation shows that two oxygen atoms are required. The coefficient 2 indicates quantity; [O] itself remains one atom.

What is an electrophile ?

An electrophile is an electron-deficient species that can accept a pair of electrons from an electron-rich species (nucleophile). Electrophiles are attracted to negative charges or electron-dense regions.

What is a free radical ?

A tertiary amine is an organic compound in which all three hydrogen atoms of ammonia have been replaced by alkyl or aryl groups, giving the structure R 3 N.

Free radical substitution is a .......... -step reaction: .......... , .......... , and .......... .

Free radical substitution is a three -step reaction: initiation , propagation , and termination .

What type of bond fission occurs in the initiation step of free radical substitution, and what provides the energy?

Homolytic fission occurs, with each atom taking one electron from the bond. The energy is provided by ultraviolet (UV) light .

True or False? In the termination step of free radical substitution, two free radicals react together to form a stable molecule.

True. Termination occurs when any two free radicals combine, removing radicals from the reaction and ending the chain. Multiple termination products are possible.

In the propagation steps of chlorination of methane, a chlorine radical reacts with methane to form a .......... radical and .......... , then the methyl radical reacts with Cl 2 to form .......... and a new chlorine radical.

In the propagation steps of chlorination of methane, a chlorine radical reacts with methane to form a methyl radical and HCl , then the methyl radical reacts with Cl 2 to form chloromethane and a new chlorine radical.

True or False? In the propagation steps of free radical substitution, the number of free radicals stays constant because one radical is consumed and one is produced in each step.

True. Each propagation step consumes one radical and produces one new radical, so the chain continues until two radicals meet in a termination step.

How many propagation steps does free radical substitution always have, and why?

Always two propagation steps: one where a radical reacts with a molecule (abstracting an atom to form a new molecule and a new radical) and one where the new radical reacts with another molecule to regenerate the original type of radical.

What is a nucleophilic substitution reaction?

A secondary amine is an organic compound in which two hydrogen atoms of ammonia have been replaced by alkyl or aryl groups, giving the structure R 2 NH.

In nucleophilic substitution, the C–X bond is polar because the halogen is more .......... than carbon, making the carbon atom .......... and susceptible to attack by a nucleophile.

In nucleophilic substitution, the C–X bond is polar because the halogen is more electronegative than carbon, making the carbon atom δ+ and susceptible to attack by a nucleophile.

What nucleophile is used to convert a halogenoalkane into a nitrile, and what is the advantage of this reaction?

The cyanide ion (CN - ) in ethanolic KCN, heated under reflux. The advantage is that it adds one carbon atom to the chain, increasing the chain length by one.

True or False? In the nucleophilic substitution mechanism, the first curly arrow must start from a lone pair on the nucleophile and point to the δ+ carbon atom.

True. The arrow must start from the lone pair (not the atom symbol) on the nucleophile and point clearly to the δ+ carbon. Arrows starting from an atom rather than a lone pair are penalised.

When excess ammonia (NH 3 ) reacts with a halogenoalkane under nucleophilic substitution, the initial product is an .......... salt, which then reacts with more NH 3 to form a .......... amine.

When excess ammonia (NH 3 ) reacts with a halogenoalkane under nucleophilic substitution, the initial product is an ammonium salt, which then reacts with more NH 3 to form a primary amine.

True or False? In nucleophilic substitution, the nucleophile should be drawn as the free ion (e.g. :OH - or :CN - ) rather than as part of a covalent compound such as NaOH.

True. Drawing NaOH (with a Na–O bond) instead of the free :OH - ion is penalised, because it is the ionic species that acts as the nucleophile.

What reagent and conditions convert a halogenoalkane into an alcohol via nucleophilic substitution?

Aqueous sodium hydroxide (NaOH(aq)) , heated. The OH - ion acts as the nucleophile, displacing the halide ion to give an alcohol.

What reagent and conditions are used to carry out elimination of a halogenoalkane?

Concentrated sodium hydroxide dissolved in ethanol (ethanolic NaOH), heated under reflux. The hydroxide ion acts as a base, removing H + from the adjacent carbon.

In elimination of bromoethane with ethanolic NaOH, a hydrogen atom is removed from the carbon .......... to the C–Br carbon, the C–H and C–Br bonds break .......... , and a C=C bond forms, giving .......... .

In elimination of bromoethane with ethanolic NaOH, a hydrogen atom is removed from the carbon adjacent to the C–Br carbon, the C–H and C–Br bonds break simultaneously , and a C=C bond forms, giving ethene .

True or False? The elimination of a halogenoalkane occurs in a single concerted step.

True. The base removes H + , the C–H bond breaks, and the C–X bond breaks simultaneously — all in one step. No carbocation intermediate is formed.

What determines whether a halogenoalkane undergoes elimination or nucleophilic substitution when treated with NaOH?

The solvent and temperature : ethanolic NaOH and heat favours elimination (giving an alkene); aqueous NaOH favours nucleophilic substitution (giving an alcohol).

In the elimination mechanism, the hydroxide ion acts as a .......... (accepting H + from the carbon adjacent to the leaving group).

In the elimination mechanism, the hydroxide ion acts as a base (accepting H + from the carbon adjacent to the leaving group).

True or False? The overall equation for elimination of bromoethane with NaOH shows NaBr and H 2 O as by-products alongside the alkene.

True. CH 3 CH 2 Br + NaOH (ethanol, heat) → CH 2 =CH 2 + NaBr + H 2 O. The halide and hydroxide combine as a salt, and water is formed from the H + and OH - .

What product is formed by elimination of 2-bromobutane with ethanolic NaOH?

But-2-ene (and but-1-ene as a minor product). Elimination removes H from a carbon adjacent to C2, forming a C=C double bond. Two positions are possible, but the more substituted alkene (but-2-ene) is usually the major product.

An elimination reaction is one in which a small molecule (such as a hydrogen halide) is removed from a larger molecule, forming a C=C double bond .

What is an electrophilic addition reaction?

An exothermic reaction is a reaction in which energy is released to the surroundings, causing the temperature of the surroundings to increase. The enthalpy change (ΔH) is negative.

When HBr reacts with ethene, the H atom acts as an .......... and accepts a pair of electrons from the C=C bond; the H–Br bond then breaks .......... , releasing Br - which attacks the .......... intermediate.

When HBr reacts with ethene, the H atom acts as an electrophile and accepts a pair of electrons from the C=C bond; the H–Br bond then breaks heterolytically , releasing Br - which attacks the carbocation intermediate.

Why does Br 2 , a non-polar molecule, act as an electrophile when it approaches an alkene?

The high electron density of the C=C bond induces a dipole in the Br 2 molecule ( induced polarity ): the nearer Br atom becomes δ+ and acts as the electrophile, while the far Br atom becomes δ− and later leaves as Br - .

True or False? When HBr adds to an unsymmetrical alkene such as propene, the major product contains bromine on the more substituted carbon.

True. The H + adds to give the more stable (more substituted) carbocation intermediate; the Br - then attacks that carbocation. The major product therefore has Br on the more substituted carbon (Markovnikov addition).

In electrophilic addition of HBr to propene, the .......... carbocation intermediate is more stable than the .......... carbocation, so the major product is .......... .

In electrophilic addition of HBr to propene, the secondary carbocation intermediate is more stable than the primary carbocation, so the major product is 2-bromopropane .

True or False? The order of carbocation stability is: tertiary > secondary > primary.

True. Alkyl groups are electron-donating, which stabilise the positive charge on the carbon. More alkyl groups attached to the carbocation carbon means greater stability.

What is the product when ethene undergoes electrophilic addition with concentrated H 2 SO 4 ?

Ethyl hydrogensulfate (CH 3 CH 2 OSO 3 H). H from H 2 SO 4 acts as the electrophile and adds to the C=C bond, forming a carbocation. HSO 4 - then acts as the nucleophile and attacks the carbocation. Adding water to ethyl hydrogensulfate then hydrolyses it to give ethanol and regenerate H 2 SO 4 .

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Organic Mechanisms (AQA A Level Chemistry): Flashcards

Exam code: 7405

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FrontFundamentals of Reaction Mechanisms
What is homolytic fission?

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What is homolytic fission?
A polyester is a condensation polymer formed by the reaction between a diol and a dicarboxylic acid (or diacid chloride), with the repeat units linked by ester bonds (–COO–).
In heterolytic fission, the more .......... atom takes both electrons from the bond, forming a .......... ion and leaving behind a .......... ion.
In heterolytic fission, the more electronegative atom takes both electrons from the bond, forming a negative ion and leaving behind a positive ion.
What is a nucleophile?
A nucleophile is an electron-rich species that can donate a pair of electrons to an electron-deficient species. Nucleophiles are attracted to positive charges or δ+ centres.
True or False?
In organic reaction mechanisms, curly arrows represent the movement of single electrons.
False.
Double-headed curly arrows represent the movement of electron pairs. Single-headed (fishhook) arrows represent single-electron movement in radical mechanisms.
A curly arrow in a mechanism begins at a .......... or a .......... pair of electrons, and points to the species that .......... the electrons.
A curly arrow in a mechanism begins at a bond or a lone pair of electrons, and points to the species that accepts the electrons.
Name the five main types of organic reaction.
1. Addition
2. Substitution
3. Elimination
4. Hydrolysis
5. Condensation
True or False?
In organic chemistry, the symbol [O] in an equation represents one atom of oxygen from an oxidising agent.
True.
[O] represents one atom of oxygen donated by an oxidising agent. When more than one oxygen atom is transferred, a coefficient is used. For example:
RCH₂OH + 2[O] → RCOOH + H₂O
The equation shows that two oxygen atoms are required. The coefficient 2 indicates quantity; [O] itself remains one atom.
What is an electrophile?
An electrophile is an electron-deficient species that can accept a pair of electrons from an electron-rich species (nucleophile). Electrophiles are attracted to negative charges or electron-dense regions.
What is a free radical?
A tertiary amine is an organic compound in which all three hydrogen atoms of ammonia have been replaced by alkyl or aryl groups, giving the structure R₃N.
Free radical substitution is a .......... -step reaction: .......... , .......... , and .......... .
Free radical substitution is a three-step reaction: initiation, propagation, and termination.
What type of bond fission occurs in the initiation step of free radical substitution, and what provides the energy?
Homolytic fission occurs, with each atom taking one electron from the bond. The energy is provided by ultraviolet (UV) light.
True or False?
In the termination step of free radical substitution, two free radicals react together to form a stable molecule.
True.
Termination occurs when any two free radicals combine, removing radicals from the reaction and ending the chain. Multiple termination products are possible.
In the propagation steps of chlorination of methane, a chlorine radical reacts with methane to form a .......... radical and .......... , then the methyl radical reacts with Cl₂ to form .......... and a new chlorine radical.
In the propagation steps of chlorination of methane, a chlorine radical reacts with methane to form a methyl radical and HCl, then the methyl radical reacts with Cl₂ to form chloromethane and a new chlorine radical.
True or False?
In the propagation steps of free radical substitution, the number of free radicals stays constant because one radical is consumed and one is produced in each step.
True.
Each propagation step consumes one radical and produces one new radical, so the chain continues until two radicals meet in a termination step.
How many propagation steps does free radical substitution always have, and why?
Always two propagation steps: one where a radical reacts with a molecule (abstracting an atom to form a new molecule and a new radical) and one where the new radical reacts with another molecule to regenerate the original type of radical.
What is a nucleophilic substitution reaction?
A secondary amine is an organic compound in which two hydrogen atoms of ammonia have been replaced by alkyl or aryl groups, giving the structure R₂NH.
In nucleophilic substitution, the C–X bond is polar because the halogen is more .......... than carbon, making the carbon atom .......... and susceptible to attack by a nucleophile.
In nucleophilic substitution, the C–X bond is polar because the halogen is more electronegative than carbon, making the carbon atom δ+ and susceptible to attack by a nucleophile.
What nucleophile is used to convert a halogenoalkane into a nitrile, and what is the advantage of this reaction?
The cyanide ion (CN^-) in ethanolic KCN, heated under reflux. The advantage is that it adds one carbon atom to the chain, increasing the chain length by one.
True or False?
In the nucleophilic substitution mechanism, the first curly arrow must start from a lone pair on the nucleophile and point to the δ+ carbon atom.
True.
The arrow must start from the lone pair (not the atom symbol) on the nucleophile and point clearly to the δ+ carbon. Arrows starting from an atom rather than a lone pair are penalised.
When excess ammonia (NH₃) reacts with a halogenoalkane under nucleophilic substitution, the initial product is an .......... salt, which then reacts with more NH₃ to form a .......... amine.
When excess ammonia (NH₃) reacts with a halogenoalkane under nucleophilic substitution, the initial product is an ammonium salt, which then reacts with more NH₃ to form a primary amine.
True or False?
In nucleophilic substitution, the nucleophile should be drawn as the free ion (e.g. :OH^- or :CN^-) rather than as part of a covalent compound such as NaOH.
True.
Drawing NaOH (with a Na–O bond) instead of the free :OH^- ion is penalised, because it is the ionic species that acts as the nucleophile.
What reagent and conditions convert a halogenoalkane into an alcohol via nucleophilic substitution?
Aqueous sodium hydroxide (NaOH(aq)), heated. The OH^- ion acts as the nucleophile, displacing the halide ion to give an alcohol.
What reagent and conditions are used to carry out elimination of a halogenoalkane?
Concentrated sodium hydroxide dissolved in ethanol (ethanolic NaOH), heated under reflux. The hydroxide ion acts as a base, removing H⁺ from the adjacent carbon.
In elimination of bromoethane with ethanolic NaOH, a hydrogen atom is removed from the carbon .......... to the C–Br carbon, the C–H and C–Br bonds break .......... , and a C=C bond forms, giving .......... .
In elimination of bromoethane with ethanolic NaOH, a hydrogen atom is removed from the carbon adjacent to the C–Br carbon, the C–H and C–Br bonds break simultaneously, and a C=C bond forms, giving ethene.
True or False?
The elimination of a halogenoalkane occurs in a single concerted step.
True.
The base removes H⁺, the C–H bond breaks, and the C–X bond breaks simultaneously — all in one step. No carbocation intermediate is formed.
What determines whether a halogenoalkane undergoes elimination or nucleophilic substitution when treated with NaOH?
The solvent and temperature: ethanolic NaOH and heat favours elimination (giving an alkene); aqueous NaOH favours nucleophilic substitution (giving an alcohol).
In the elimination mechanism, the hydroxide ion acts as a .......... (accepting H⁺ from the carbon adjacent to the leaving group).
In the elimination mechanism, the hydroxide ion acts as a base (accepting H⁺ from the carbon adjacent to the leaving group).
True or False?
The overall equation for elimination of bromoethane with NaOH shows NaBr and H₂O as by-products alongside the alkene.
True.
CH₃CH₂Br + NaOH (ethanol, heat) → CH₂=CH₂ + NaBr + H₂O. The halide and hydroxide combine as a salt, and water is formed from the H⁺ and OH^-.
What product is formed by elimination of 2-bromobutane with ethanolic NaOH?
But-2-ene (and but-1-ene as a minor product). Elimination removes H from a carbon adjacent to C2, forming a C=C double bond. Two positions are possible, but the more substituted alkene (but-2-ene) is usually the major product.
Elimination reaction
An elimination reaction is one in which a small molecule (such as a hydrogen halide) is removed from a larger molecule, forming a C=C double bond.
What is an electrophilic addition reaction?
An exothermic reaction is a reaction in which energy is released to the surroundings, causing the temperature of the surroundings to increase. The enthalpy change (ΔH) is negative.
When HBr reacts with ethene, the H atom acts as an .......... and accepts a pair of electrons from the C=C bond; the H–Br bond then breaks .......... , releasing Br^- which attacks the .......... intermediate.
When HBr reacts with ethene, the H atom acts as an electrophile and accepts a pair of electrons from the C=C bond; the H–Br bond then breaks heterolytically, releasing Br^- which attacks the carbocation intermediate.
Why does Br₂, a non-polar molecule, act as an electrophile when it approaches an alkene?
The high electron density of the C=C bond induces a dipole in the Br₂ molecule (induced polarity): the nearer Br atom becomes δ+ and acts as the electrophile, while the far Br atom becomes δ− and later leaves as Br^-.
True or False?
When HBr adds to an unsymmetrical alkene such as propene, the major product contains bromine on the more substituted carbon.
True.
The H⁺ adds to give the more stable (more substituted) carbocation intermediate; the Br^- then attacks that carbocation. The major product therefore has Br on the more substituted carbon (Markovnikov addition).
In electrophilic addition of HBr to propene, the .......... carbocation intermediate is more stable than the .......... carbocation, so the major product is .......... .
In electrophilic addition of HBr to propene, the secondary carbocation intermediate is more stable than the primary carbocation, so the major product is 2-bromopropane.
True or False?
The order of carbocation stability is: tertiary > secondary > primary.
True.
Alkyl groups are electron-donating, which stabilise the positive charge on the carbon. More alkyl groups attached to the carbocation carbon means greater stability.
What is the product when ethene undergoes electrophilic addition with concentrated H₂SO₄?
Ethyl hydrogensulfate (CH₃CH₂OSO₃H). H from H₂SO₄ acts as the electrophile and adds to the C=C bond, forming a carbocation. HSO₄^- then acts as the nucleophile and attacks the carbocation.
Adding water to ethyl hydrogensulfate then hydrolyses it to give ethanol and regenerate H₂SO₄.

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