* Topic 6 Part 2 - Halogenoalkanes + alcohols Flashcards
How can halogenoalkanes be classified by and how is this decided
Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group.
What types of reactions can halogenoalkanes undergo
Substitution or elimination
What is a nucleophilic substitution reaction
swapping a halogen atom for another atom or groups of atoms
What is a nucleophile
electron pair donator e.g. :OH-, :NH3, CN-
What does :Nu represent
:Nu represents any nucleophile – they always have a lone pair and act as electron pair donators
What do the nucleophiles attack in a nucleophilic substitution mechanism
The nucleophiles attack the positive carbon atom
Why does the carbon have a small positive charge in the first step of a nucleophilic substitution reaction
The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen
What do the curly arrows in mechanisms represent
We use curly arrows in mechanisms (with two line heads) to show the movement of two electrons
Describe the movement of a curly arrow in a mechanism (where does it begin and end)
A curly arrow will always start from a lone pair of electrons or the centre of a bond
What does the rate of nucleophilic substitution reactions depend on
The rate of these substitution reactions depends on the strength of the C-X bond
The weaker the bond, the easier it is to break and the faster the reaction.
Which halogenoalkane is the fastest to substitute and which is the slowest
The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the C-F bond is such that fluoroalkanes are very unreactive
What is hydrolysis
Hydrolysis is defined as the splitting of a molecule ( in this case a halogenoalkane) by a reaction with water
Comment on the reactivity of water as a nucleophile
Water is a poor nucleophile but it can react slowly with halogenoalkanes in a substitution reaction
Give the equation (using X as the halogen) of the hydrolysis of a halogenoalkane
CH3CH2X + H2O -> CH3CH2OH + X- + H+
What test can be used to compare the reactivity of the different halogenoalkanes
Aqueous silver nitrate is added to a halogenoalkane and the halide leaving group combines with a silver ion to form a silver halide precipitate.
The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different halogenoalkanes.
Which halogen forms the weakest C-X bond in a halogenoalkane
Iodine
Halogenoalkane -> alcohol
Reagent
Conditions
Mechanism
Role of reagent
Example - 1-bromopropane
Reagent - potassium (or sodium) hydroxide
Conditions - In AQUEOUS solution; heat under reflux
Mechanism - Nucleophilic substitution
Role of reagent - Nucleophile, OH-
1-bromopropane +KOH -> propan-1-ol + KBr
Why is it important for the reaction of a halogenoalkane to an alcohol that the conditions be aqueous
If the solvent is changed to ethanol an elimination reaction occurs
Why is OH- a stronger nucleophile than water
The OH– is a stronger nucleophile than water as it has a full negative charge and so is more strongly attracted to the Cδ+
What is the difference between SN1 and SN2
SN1 - nucleophilic substitution mechanism for tertiary halogenoalkanes
SN2 - nucleophilic substitution mechanism for (primary) halogenoalkanes
Why is SN1 able to occur in tertiary halogenoalkanes and not in primary halogenoalkanes
Tertiary halogenoalkanes undergo this mechanism as the tertiary carbocation is stabilised by the electron releasing methyl groups around it. (See alkenes topic for another example of this). Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane in the same way as the mechanism above
Primary halogenoalkanes do not do the SN1 mechanism because they would only form an unstable primary carbocation.
What are the steps of SN1 in 2-bromo-2-methylpropane
The Br first breaks away from the halogenoalkane to form a carbocation intermediate
The hydroxide nucleophile then attacks the positive carbon
Draw the SN1 mechanism for a tertiary halogenoalkane (eg. 2-bromo-2-methylpropane)
Draw the SN2 mechanism for a primary halogenoalkane (eg. 1-bromoethane)
