Topic 20.1: Organic chemistry – Type of organic reactions
Key organic reaction types include nucleophilic substitution, electrophilic addition, electrophilic substitution and redox reactions. Reaction
mechanisms vary and help in understanding the different types of reaction taking place.
• Understanding: SN1 represents a nucleophilic unimolecular substitution reaction and SN2 represents a nucleophilic bimolecular
substitution reaction. SN1 involves a carbocation intermediate. SN2 involves a concerted reaction with a transition state.
▪ Halogenoalkanes: reactive due to high electronegativity difference and polar carbon-halogen bonds
• Partial positive charge makes the carbon atoms electron deficient and susceptible to attack by nucleophiles
Characteristics SN1 reaction SN2 reaction
Molecularity Unimolecular Bimolecular
Reaction Carbocation intermediate Transition state (concerted)
Types of halogenoalkanes Tertiary Primary
▪ Concerted reaction: single step reaction through which reactants are converted directly into products with no intermediates
• Understanding: For tertiary halogenoalkanes the predominant mechanism is SN1 and for primary halogenoalkanes it is SN2. Both
mechanisms occur for secondary halogenoalkanes.
▪ SN1 reaction in tertiary halogenoalkanes
• Inductive effect: alkane groups bonded to a carbocation have a positive inductive effect that reducing the positive
charge on the carbocation by donating electron density and stabilizes molecule
▪ SN2 reaction in primary halogenoalkanes
• Inductive effect: lack of alkane groups bonded to a carbocation does not result in inductive effect that causes the
positive charge on the carbocation to remain and stay as an unstable, strong electrophile
• Understanding: The rate determining step (slow step) in an SN1 reaction depends only on the concentration of the halogenoalkane,
rate = k[halogenoalkane]. For SN2, rate = k[halogenoalkane][nucleophile]. SN2 is stereospecific with an inversion of configuration
at the carbon.
▪ Rate of SN1 reaction: rate = k[halogenoalkane]
• Rate determining step: SN1 reaction is carried in two step, and RDS involves only the leaving group
▪ Rate of SN2 reaction: rate = k[halogenoalkane][nucleophile]
• Rate determining step: SN2 reaction is carried in one step, so both halogenoalkane and nucleophiles are involved
• Inversion of configuration: steric hindrance causes “backside attack” of nucleophiles, causing a Walden inversion
• Stereospecific: stereochemistry of the product is determined completely by the stereochemistry of the reaction
• Understanding: SN2 reactions are best conducted using aprotic, non-polar solvents and SN1 reactions are best conducted using
protic, polar solvents.
• Applications and skills: Outline of the difference between protic and aprotic solvents.
▪ Ideal SN1 solvent condition
• Protic, polar solvents: polar solvent that contains a hydrogen atom bonded to a nitrogen or oxygen atom (cause
intermolecular hydrogen bonding)
• Effect: protic solvent form a solvation shell around nucleophile preventing them from attack electrophiles
• This prevents nucleophiles from being effective in forming a SN2 transition intermediate
▪ Ideal SN2 solvent condition
• Aprotic, polar solvents: polar solvent that does not contain hydrogen atoms bonded to nitrogen or oxygen atoms (does
not cause intermolecular hydrogen bonding)
• Effect: aprotic solvent cannot solvate nucleophile maintaining its property as a strong nucleophile
• Understanding: An electrophile is an electron-deficient species that can accept electron pairs from a nucleophile. Electrophiles are
Lewis acids.
▪ Electrophile: electron-deficient species that can accept electron pairs from a nucleophile; are Lewis acids
• Characteristics: have either a formal positive charge or a partial positive charge due to electronegativity difference
• Understanding: Markovnikov’s rule can be applied to predict the major product in electrophilic addition reactions of
unsymmetrical alkenes with hydrogen halides and interhalogens. The formation of the major product can be explained in terms of
the relative stability of possible carbocations in the reaction mechanism.
▪ Markovnikov’s rule: states that hydrogen atom will preferentially bond to the carbon atom of the alkene with the largest number
of hydrogen substituents in a unsymmetrical double bond
• Addition of hydrogen ion to carbon atom creates positive charge on the other carbon forming a carbocation
• The more alkyl group attached to carbocation, the more stable it is due to induction and become more likely to from
• Major products (stable halogenoalkane;): forms when the hydrogen atom is bonded to carbon that are less substituted
by alkyl groups
• Minor products (unstable halogenoalkane; i.e. primary): forms when the hydrogen atom is bonded to a carbon that are
more substituted by alkyl group
, • Understanding: Benzene is the simplest aromatic hydrocarbon compound (or arene) and has a delocalized structure of π bonds
around its ring. Each carbon to carbon bond has a bond order of 1.5. Benzene is susceptible to attack by electrophiles.
▪ Benzene: simplest aromatic hydrocarbon compound that has a delocalized structure of π bonds around its ring
• Carbon-carbon bond order: bond order of 1.5; intermediate between single and double bond
• Structure: delocalized electrons that are not specific to a certain atom in the molecule
▪ Electrophilic substitution reaction: delocalized electrons from benzene attracts electrophiles, even though it does not go under
addition reaction due to the stability of the aromatic ring
• Understanding: Carboxylic acids can be reduced to primary alcohols (via the aldehyde). Ketones can be reduced to secondary
alcohols. Typical reducing agents are lithium aluminium hydride (used to reduce carboxylic acids) and sodium borohydride.
Primary alcohol Secondary alcohol
Stages Carboxylic acid → aldehyde → alcohol ketone → alcohol
Diagram
Condition High temperature High temperature
Catalyst Lithium aluminium hydride – LiAlH4 Lithium aluminium hydride – LiAlH4
Sodium borohydride – NaBH4 Sodium borohydride – NaBH4
▪ Distillation: separation of mixture based on different boiling points
▪ Reflux: cyclic evaporation and condensation of a reaction mixture to prevent solvent from evaporating and keep it in the system
• Used if only the final product is desired
• Applications and skills: Explanation of why hydroxide is a better nucleophile than water.
Characteristics Hydroxide Water
Formula OH- H2O
Base property Lewis base (contains at least one lone pair) Lewis base (contains at least one lone pair)
Charge Negative Neutral
Attraction Strong due to charge Moderate due to charge
• Applications and skills: Deduction of the mechanism of the nucleophilic substitution reactions of halogenoalkanes with aqueous
sodium hydroxide in terms of SN1 and SN2 mechanisms. Explanation of how the rate depends on the identity of the halogen (ie the
leaving group), whether the halogenoalkane is primary, secondary or tertiary and the choice of solvent.
▪ SN1 substitution mechanism
• Curly arrow: originates from bond between carbon
and halogen and terminates at the halogen group
• Heterolytic fission: bond breaks and the electron pair
is passed onto the more electronegative atom
• Carbocation formation: positive charge ins centred on
the carbon atom
• Curly arrow: originates from the lone pair or negative
charge of nucleophile and terminates at the carbon atom
▪ SN2 substitution mechanism
• Curly arrow: originates from the lone pair or negative
charge of nucleophile and terminates at the carbon atom
• Curly arrow: originates from bond between carbon and
halogen and terminates at the halogen group
• Transition state: partial bonds are indicated by dotted
lines and square brackets with single negative charge
Factors Description
Identity of halogen The quicker the halogen can leave, the quicker the rate of reaction; weak bond strength and low
electronegativity difference between carbon and halogen results in quicker breaking of bonds
Identity of halogenoalkane SN1 reaction: tertiary (and secondary) halogenoalkane
SN2 reaction: primary (and secondary) halogenoalkane
Identity of solvent SN1 reaction: polar, protic solvent
SN2 reaction: polar, aprotic solvent
Key organic reaction types include nucleophilic substitution, electrophilic addition, electrophilic substitution and redox reactions. Reaction
mechanisms vary and help in understanding the different types of reaction taking place.
• Understanding: SN1 represents a nucleophilic unimolecular substitution reaction and SN2 represents a nucleophilic bimolecular
substitution reaction. SN1 involves a carbocation intermediate. SN2 involves a concerted reaction with a transition state.
▪ Halogenoalkanes: reactive due to high electronegativity difference and polar carbon-halogen bonds
• Partial positive charge makes the carbon atoms electron deficient and susceptible to attack by nucleophiles
Characteristics SN1 reaction SN2 reaction
Molecularity Unimolecular Bimolecular
Reaction Carbocation intermediate Transition state (concerted)
Types of halogenoalkanes Tertiary Primary
▪ Concerted reaction: single step reaction through which reactants are converted directly into products with no intermediates
• Understanding: For tertiary halogenoalkanes the predominant mechanism is SN1 and for primary halogenoalkanes it is SN2. Both
mechanisms occur for secondary halogenoalkanes.
▪ SN1 reaction in tertiary halogenoalkanes
• Inductive effect: alkane groups bonded to a carbocation have a positive inductive effect that reducing the positive
charge on the carbocation by donating electron density and stabilizes molecule
▪ SN2 reaction in primary halogenoalkanes
• Inductive effect: lack of alkane groups bonded to a carbocation does not result in inductive effect that causes the
positive charge on the carbocation to remain and stay as an unstable, strong electrophile
• Understanding: The rate determining step (slow step) in an SN1 reaction depends only on the concentration of the halogenoalkane,
rate = k[halogenoalkane]. For SN2, rate = k[halogenoalkane][nucleophile]. SN2 is stereospecific with an inversion of configuration
at the carbon.
▪ Rate of SN1 reaction: rate = k[halogenoalkane]
• Rate determining step: SN1 reaction is carried in two step, and RDS involves only the leaving group
▪ Rate of SN2 reaction: rate = k[halogenoalkane][nucleophile]
• Rate determining step: SN2 reaction is carried in one step, so both halogenoalkane and nucleophiles are involved
• Inversion of configuration: steric hindrance causes “backside attack” of nucleophiles, causing a Walden inversion
• Stereospecific: stereochemistry of the product is determined completely by the stereochemistry of the reaction
• Understanding: SN2 reactions are best conducted using aprotic, non-polar solvents and SN1 reactions are best conducted using
protic, polar solvents.
• Applications and skills: Outline of the difference between protic and aprotic solvents.
▪ Ideal SN1 solvent condition
• Protic, polar solvents: polar solvent that contains a hydrogen atom bonded to a nitrogen or oxygen atom (cause
intermolecular hydrogen bonding)
• Effect: protic solvent form a solvation shell around nucleophile preventing them from attack electrophiles
• This prevents nucleophiles from being effective in forming a SN2 transition intermediate
▪ Ideal SN2 solvent condition
• Aprotic, polar solvents: polar solvent that does not contain hydrogen atoms bonded to nitrogen or oxygen atoms (does
not cause intermolecular hydrogen bonding)
• Effect: aprotic solvent cannot solvate nucleophile maintaining its property as a strong nucleophile
• Understanding: An electrophile is an electron-deficient species that can accept electron pairs from a nucleophile. Electrophiles are
Lewis acids.
▪ Electrophile: electron-deficient species that can accept electron pairs from a nucleophile; are Lewis acids
• Characteristics: have either a formal positive charge or a partial positive charge due to electronegativity difference
• Understanding: Markovnikov’s rule can be applied to predict the major product in electrophilic addition reactions of
unsymmetrical alkenes with hydrogen halides and interhalogens. The formation of the major product can be explained in terms of
the relative stability of possible carbocations in the reaction mechanism.
▪ Markovnikov’s rule: states that hydrogen atom will preferentially bond to the carbon atom of the alkene with the largest number
of hydrogen substituents in a unsymmetrical double bond
• Addition of hydrogen ion to carbon atom creates positive charge on the other carbon forming a carbocation
• The more alkyl group attached to carbocation, the more stable it is due to induction and become more likely to from
• Major products (stable halogenoalkane;): forms when the hydrogen atom is bonded to carbon that are less substituted
by alkyl groups
• Minor products (unstable halogenoalkane; i.e. primary): forms when the hydrogen atom is bonded to a carbon that are
more substituted by alkyl group
, • Understanding: Benzene is the simplest aromatic hydrocarbon compound (or arene) and has a delocalized structure of π bonds
around its ring. Each carbon to carbon bond has a bond order of 1.5. Benzene is susceptible to attack by electrophiles.
▪ Benzene: simplest aromatic hydrocarbon compound that has a delocalized structure of π bonds around its ring
• Carbon-carbon bond order: bond order of 1.5; intermediate between single and double bond
• Structure: delocalized electrons that are not specific to a certain atom in the molecule
▪ Electrophilic substitution reaction: delocalized electrons from benzene attracts electrophiles, even though it does not go under
addition reaction due to the stability of the aromatic ring
• Understanding: Carboxylic acids can be reduced to primary alcohols (via the aldehyde). Ketones can be reduced to secondary
alcohols. Typical reducing agents are lithium aluminium hydride (used to reduce carboxylic acids) and sodium borohydride.
Primary alcohol Secondary alcohol
Stages Carboxylic acid → aldehyde → alcohol ketone → alcohol
Diagram
Condition High temperature High temperature
Catalyst Lithium aluminium hydride – LiAlH4 Lithium aluminium hydride – LiAlH4
Sodium borohydride – NaBH4 Sodium borohydride – NaBH4
▪ Distillation: separation of mixture based on different boiling points
▪ Reflux: cyclic evaporation and condensation of a reaction mixture to prevent solvent from evaporating and keep it in the system
• Used if only the final product is desired
• Applications and skills: Explanation of why hydroxide is a better nucleophile than water.
Characteristics Hydroxide Water
Formula OH- H2O
Base property Lewis base (contains at least one lone pair) Lewis base (contains at least one lone pair)
Charge Negative Neutral
Attraction Strong due to charge Moderate due to charge
• Applications and skills: Deduction of the mechanism of the nucleophilic substitution reactions of halogenoalkanes with aqueous
sodium hydroxide in terms of SN1 and SN2 mechanisms. Explanation of how the rate depends on the identity of the halogen (ie the
leaving group), whether the halogenoalkane is primary, secondary or tertiary and the choice of solvent.
▪ SN1 substitution mechanism
• Curly arrow: originates from bond between carbon
and halogen and terminates at the halogen group
• Heterolytic fission: bond breaks and the electron pair
is passed onto the more electronegative atom
• Carbocation formation: positive charge ins centred on
the carbon atom
• Curly arrow: originates from the lone pair or negative
charge of nucleophile and terminates at the carbon atom
▪ SN2 substitution mechanism
• Curly arrow: originates from the lone pair or negative
charge of nucleophile and terminates at the carbon atom
• Curly arrow: originates from bond between carbon and
halogen and terminates at the halogen group
• Transition state: partial bonds are indicated by dotted
lines and square brackets with single negative charge
Factors Description
Identity of halogen The quicker the halogen can leave, the quicker the rate of reaction; weak bond strength and low
electronegativity difference between carbon and halogen results in quicker breaking of bonds
Identity of halogenoalkane SN1 reaction: tertiary (and secondary) halogenoalkane
SN2 reaction: primary (and secondary) halogenoalkane
Identity of solvent SN1 reaction: polar, protic solvent
SN2 reaction: polar, aprotic solvent
