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Advanced Level Organic Chemistry: Halogenoalkanes: hydrolysis reaction with water

Part 3. The chemistry of HALOGENOALKANES

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

3.3 Introduction to the nucleophilic substitution reactions of halogenoalkanes, the substitution reaction and hydrolysis with water and experiments with silver nitrate solution to investigate reactivity trends and introducing SN1 and SN2 mechanistic pathways

Hydrolysis of haloalkanes with NaOH/KOH is described in Part 3.4

Halogenoalkanes were once known as 'haloalkanes' or 'alkyl halides', but the correct IUPAC nomenclature is based on calling halogenated alkanes halogenoalkanes. However, it seems ok to refer to chloroalkanes, bromoalkanes and iodoalkanes. I've written the equations for the reactions showing the formation of an alcohol from the halogenoalkane in multiple styles and added the nucleophilic substitution mechanisms where appropriate.

Sub-index for this page

3.3.1 Why do halogenoalkanes undergo nucleophilic substitution reactions?, why are haloalkanes more reactive than alkanes?, what are their reactivity trends? and note on sub-classification

3.3.2 The hydrolysis reaction between a tertiary halogenoalkane and pure water - an SN1 mechanism

3.3.3 Hydrolysis of halogenoalkanes with pure water - investigating relative reactivity trends with AgNO3

3.3.4 The hydrolysis reaction of a primary haloalkane with pure water - an SN2 mechanism

3.3.5 The hydrolysis of secondary halogenoalkanes with pure water

3.3.6 SN1 and SN2 water hydrolysis mechanisms, rate expressions, orders of reaction

See also

My original detailed discussion of the nucleophilic substitution mechanism between a halogenoalkane and water/hydroxide ion

HALOGENOALKANES chemistry notes INDEX

All Advanced A Level Organic Chemistry Notes

Index of basic Oil and Organic Chemistry Revision Notes

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I've often added the boiling point (bpt) so can see what is a liquid and could be hydrolysed in a school/college laboratory.

Strictly speaking all the reactants and products should be suffixed by (aq) apart fro water (l).



3.3.1 Why do halogenoalkanes (RX) undergo nucleophilic substitution reactions?

diagram structure of halogenoalkane haloalkane functional group general structural formula advanced level organic chemistry

You must know the structures of primary, secondary and tertiary halogenoalkanes (haloalkanes)

e.g. hydrolysis with water or alkali to form an alcohol: RX  +  H2O  ===>  ROH  +  X-  +  H+

Why are haloalkanes more reactive than alkanes?

and what are their reactivity trends, plus a note on sub-classification reactivity and mechanism mode

Structure and reactivity:

R3C-X = halogenoalkane (haloalkane, alkyl halide, halogenated alkane etc.)

R = H, alkyl or aryl  and X = group 7/17 halogen F, Cl, Br or I

Halogenoalkanes owe their reactivity, especially compared to the unreactive alkanes, to two principal reasons:

Three trends need to be discussed before getting into the detailed chemistry

(1) Bond polarity

The carbon-halogen bond is polar, Cδ+-Xδ- due to the difference in electronegativity between carbon and the halogen.

The Cδ+ carbon is then susceptible to nucleophilic attack by electron pair donor neutral molecules e.g. the nucleophiles :NH3  and  H2O: or nucleophile negative ions like. :OH-  and  :CN-).

This is the principal reason why halogenoalkanes undergo nucleophilic substitution reactions.

The order of bond polarity is: C-F  >  C-Cl  >  C-Br  >  C-I  because of the decreasing electronegativity of the halogen atom down group 7/17.

BUT, contrary to what you might think, the reactivity trend towards nucleophilic attack is the complete opposite, and generally speaking the reactivity order is:  R-I  >  R-Br  >  R-Cl  >  RF  because of the trend in decreasing bond enthalpy which overrides bon polarity trend - explained in the next section (ii) below.

(2) Bond enthalpies and reactivity trend

The carbon-halogen bond is usually the weakest bond in the molecule and significantly weaker than the carbon-carbon or carbon-hydrogen bonds.

Average bond enthalpies/kJmol-1: C-C  348, C-H  412, both requiring relatively high activation energies for reaction.

Average bond enthalpies/kJmol-1: C-F 484, C-Cl  338, C-Br  276, C-I  238, generally lower than C-C or C-H bonds resulting in lower activation energies.

On descending the Group 7/17 halogens, the atomic radius increases, C-X bond length increases and the strength of the bond decreases.

Even the lowering of the bond enthalpy by 10kJ from C-C to C-Cl, combined with the polarity of the C-Cl bond, makes all the difference when comparing alkane and halogenoalkane reactivity.

Note that even though the most polar bond, based on differences in electronegativity, is the carbon - fluorine bond (Cδ+-Fδ-), the bond enthalpy is so high that fluoroalkanes have the lowest reactivity of all halogenoalkanes.

So what you find in practice is the reactivity trend of R-I  >  R-Br  >  R-Cl  >  RF , that completely overrides the bond polarity trend ...

... entirely due to the bond enthalpy trend:  C-F  >  C-Cl  >  C-Br  >  C-I  ....

... and the resulting activation energy trend will be in the same order i.e. decreasing from C-F to C-I,

... the lower the activation energy, the faster the reaction under the same conditions of temperature, concentration, reagent and solvent.

(3) A note on the structural sub-classification of halogenoalkanes and its effect on reactivity

You should understand the sub-classification of halogenoalkanes before studying their reactions.

The sub-classification of halogenoalkanes is based on structural differences, which can have significant chemical consequences e.g. on the rate of the reaction, mechanism of the reaction or the products formed in the reaction.

Halogenoalkanes are classified according to the atoms/groups attached to the carbon of the halogen atom X.

Primary halogenoalkanes have the structure R-CH2-X, R = H, alkyl, aryl etc.

Apart from the likes of the methyl haloalkanes e.g. chloromethane (CH3Cl) they have two hydrogen atoms and one alkyl/aryl group attached to the C of the C-X functional group.

e.g. chloroethane CH3CH2Cl, and  1-bromopropane

C6H5CH2Cl is an example of an aryl (aromatic) substituent attached to the carbon of the C-X bond, but it is classed as a primary halogenoalkane (as well as an aromatic compound, but not classed as an aryl halide because the chlorine atom is not directly attached to the benzene ring.)

Secondary halogenoalkanes have the structure R2CH-X, R = alkyl or aryl etc.

They have one hydrogen and two alkyl/aryl groups attached to the C of the C-X functional group.

e.g. 2-iodobutane CH3CHICH2CH3 , and (c) doc b 2-chloropropane (c) doc b

Tertiary halogenoalkanes have the structure R3C-X, R = alkyl or aryl etc.

They have three alkyl/aryl groups attached to the C of the C-X group.

e.g. 2-chloro-2-methylpropane (CH3)3CI (c) doc b (c) doc b

There no hydrogen atoms on the C of the C-X functional group.

 

For the same halogen atom, generally speaking the reactivity trends for haloalkanes is:

tertiary  >  secondary  >  primary

e.g. if take the structural isomers of C4H9Cl, the reactivity trend would be:

(c) doc b  >  (c) doc b  >   (c) doc b  ~ (c) doc b

For the reactivity trend: tertiary  >   secondary  >  primary ~ primary halogenoalkanes.

The explanation for this is covered in the next few sections on this page when discussing the reaction mechanisms.

 

I have also included brief descriptions of how to do experiments to investigate the two most important reactivity trends based on:

(i) The nature of the halogen X in the C-X bond, the bond enthalpy based reactivity trend.

(ii) The sub-classification of the structure of the original halogenoalkane compared to its reactivity for the same halogen X in the C-X bond.

 

I've often added the boiling point (bpt) so can see what is a liquid and could be hydrolysed in a school/college laboratory.

Strictly speaking all the reactants and products should be suffixed by (aq) apart from water (l).

 

Remember when studying the reaction mechanisms in the next sections:

Remember: A neutral or negative nucleophile, Nuc: or Nuc:-, is an electron pair donor that can attack an electron deficient partially/wholly (δ+/+) positive carbon atom to form a new C-Nuc (C:Nuc) bond and displace an atom/group in the process

e.g. for the hydrolysis of halogenoalkanes in pure water to form alcohols:

 R-X(l)  +  H2O:(l)  ===>  R-OH(aq)  +  H+(aq)  +  X-(aq)   or more correctly ...

 R-X(l)  +  2H2O:(l)  ===>  R-OH(aq)  +  H3O+(aq)  +  X-(aq)

is a typical haloalkane nucleophilic substitution reaction, where water is the nucleophile,

and it is also described as a hydrolysis, because the organic molecule reacts with water to give at least two products.

Where R = alkyl, H2O: is the nucleophile - electron pair donor (: on the O), X = halogen replaced and X- is the displaced atom/group, in this case a halide ion, which is sometimes referred to as the 'leaving group'.

The mechanisms are discussed here in Part 3.3.3  and  3.3.4

The hydroxide ion (:OH-) is a much stronger nucleophile than water, since it carries a negative charge, but I've written up hydrolysis of halogenoalkanes with strong alkalis is in Part 3.4

3.4 The substitution reaction of halogenoalkanes (haloalkanes) with sodium/potassium hydroxide to give alcohols

See also Introduction to organic chemical reaction mechanisms & technical terms explained

and Nucleophilic substitution by water/hydroxide ion [SN1 or SN2, hydrolysis to give alcohols]

with extra notes on kinetics, rds, molecularity, rate expression, activated complex etc.


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3.3.2 Hydrolysis reaction between a tertiary halogenoalkane and water - SN1 mechanism

2-chloro-2-methylpropane (bpt 51oC) is an example of a halogenoalkane that readily hydrolyses in water to give the appropriate tertiary alcohol and dilute hydrochloric acid (HCl, but fully ionised!).

2-chloro-2-methylpropane  +  water  ===>  2-methylpropan-2-ol  +  hydrochloric acid

(c) doc b  +  H2  alcohols and ether structure and naming (c) doc b  +  H+  +  Cl   (structured formula equation)

(c) doc b  +  H2  alcohols and ether structure and naming (c) doc b  +  H+  +  Cl   (abbreviated structured formula equation)

(c) doc b  +  H2  alcohols and ether structure and naming (c) doc b  +  H+  +  Cl   (skeletal formula equation)

Note that a tertiary halogenoalkane on hydrolysis gives a tertiary alcohol.

 

It is a similar reaction with 2-bromo-2-methylpropane (bpt 73oC)

hydrolysis conversion:   alcohols and ether structure and naming (c) doc b   (structured formulae change)

2-bromo-2-methylpropane  +  water  ===>  2-methylpropan-2-ol  +  hydrobromic acid

(CH3)3CBr  +  H2   (CH3)3COH  +  H+  +  Br

or more correctly: (CH3)3CBr  +  2H2   (CH3)3COH  +  H3O+  +  Br

 

The general mechanism for this particular reaction is shown below in mechanism diagram 10 for Cl, but same for Br or I.

SN1 mechanism hydrolysis with water tertiary halogenoalkane haloalkane carbocation intermediate advanced organic chemistry notes doc brown

mechanism 10 - nucleophilic substitution of a halogenoalkane by water (SN1 unimolecular via carbocation)

This is the typical SN1 mechanism by which tertiary halogenoalkanes hydrolyse

What we mean by an SN1 nucleophilic substitution reaction.

The S signifies substitution, N signifies nucleophilic (attack) and the 1 means a unimolecular step 1 that also determines the rate of the reaction because the heterolytic bond fission of the C-Cl bond is the slowest step and only one molecule involved - hence the phrase 'unimolecular'.

Step (1) Heterolytic bond fission to create a carbocation and a chloride ion.

Heterolytic bond fission means the bonding pair of electrons goes completely to one of the atoms of the original bond, therefore automatically creating a positive and negative ion.

Step (2) A water molecule combines with the carbocation to give a protonated alcohol molecule.

Step (3) A proton is lost from the protonated alcohol molecule to give the alcohol e.g. 2-methylpropan-2-ol.

If R = CH3, then above is the mechanism for the hydrolysis of 2-chloro-2-methylpropane, a tertiary haloalkane.

Note that step (1) is reversible.

e.g. if you mix 2-methylpropan-2-ol with concentrated hydrochloric acid you form 2-chloro-2-methylpropane !!!

(c) doc b  +  H2  alcohols and ether structure and naming (c) doc b  +  H+  +  Cl

 

The reaction progress profile for the hydrolysis of a tertiary halogenoalkane with water via the SN1 carbocation mechanism - unimolecular rate determining step

 Reaction progress profile for SN1 unimolecular hydrolysis by water of halogenoalkanes haloalkane advanced organic chemistry notes doc brown

Generalised reaction profiles showing the formation of the intermediate carbocation.

The heterolytic bond fission to generate the carbocation has by far the largest activation energy Ea1, so unimolecular step 1 is the rate determining step.

 

The mechanism for the hydrolysis of 2-bromo-2-methylpropane is shown below in mechanism diagram 71b below:

SN1 mechanism hydrolysis of 2-bromo-2-methylpropane with water tertiary halogenoalkane carbocation intermediate advanced organic chemistry notes doc brown

Step (1) Heterolytic bond fission to create a tertiary carbocation and a bromide ion - the slow rate determining step.

Here, heterolytic bond fission of the 2-bromo-2-methylpropane means the bonding pair of electrons goes completely to the bromine atom, automatically creating a positive carbocation and a negative bromide ion.

Step (2) A water molecule combines with the carbocation to give protonated 2-methylpropan-2-ol

Step (3) A proton is lost from the protonated alcohol to a water molecule leaving the free alcohol 2-methylpropan-2-ol.

Overall reaction: (CH3)3CBr  +  2H2   (CH3)3COH  +  H3O+  +  Br

 


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3.3.3 Hydrolysis of halogenoalkanes with pure water - investigating relative reactivity trends

Primary halogenoalkanes are quite slow to hydrolyse in pure water (see Part 3.3.4 for SN2 mechanism).

To add some more skeletal formulae equations

Trend 1 The relative reactivity of the C-X bond, where X = Cl, Br and I

You make up equal concentration mixtures of the halogenoalkane, ethanol and aqueous silver nitrate.

As the halogenoalkane hydrolyses, the freed halide ion forms a precipitate with silver nitrate.

You may need to warm all the solutions to 50-60oC in a thermostated water bath to effect all three reactions.

You note the time the precipitate is first clearly visible from the reaction: Ag+(aq)  +  X-(aq)  ===>  AgX(s)

For a 'fair test' all the haloalkanes should be all of the same class i.e. in this case, all primary haloalkanes.

(a) 1-chlorobutane (bpt 79oC)  +  water  ===>  butan-1-ol  +  hydrochloric acid

(c) doc b  +  H2   CH3CH2CH2CH2OH  +  H+  +  Cl  (structured formula equation)

(c) doc b  +  H2O    alcohols and ether structure and naming (c) doc b  +  H+  +  Cl   (skeletal formula equation)

This reaction is very slow because of the strong C-Cl bond, but eventually a white precipitate of silver chloride forms.

Ag+(aq)  +  Cl-(aq)  ===>  AgCl(s)

 

(b) 1-bromobutane (bpt 101oC)  +  water  ===>  butan-1-ol  +  hydrobromic acid

CH3CH2CH2CH2Br  +  H2   CH3CH2CH2CH2OH  +  H+  +  Br

This reaction is faster, but still takes some tine, but eventually a cream precipitate of silver bromide forms.

Ag+(aq)  +  Br-(aq)  ===>  AgBr(s)

 

(c) 1-iodobutane (bpt 130oC)  +  water  ===>  butan-1-ol  +  hydroiodic acid

CH3CH2CH2CH2I  +  H2   CH3CH2CH2CH2OH  +  H+  +  I

This reaction the fastest, but still takes time, and a pale yellow precipitate of silver iodide forms.

Ag+(aq)  +  I-(aq)  ===>  AgI(s)

 

Although a very simple experiment, reactions (a) to (c) clearly demonstrate the difference in reactivity of the carbon - halogen bond

 i.e. the reactivity order of C-I  >  C-Br  >  C-I,

and from which you can infer the relative reactivity of C-F and C-At haloalkanes.

The explanation based on bond enthalpy trend is discussed in the first section 3.3.1

If you can do the experiment with the equivalent secondary haloalkanes, things go a bit faster (*).

e.g. reactivity order for 2-haloalkanes:  CH3CH2CHICH3  >  CH3CH2CHBrCH3  >  CH3CH2CHClCH3

For a fair comparison test, you should try to keep the structures as similar as possible e.g. all primary secondary or tertiary, and just vary the halogen atom.

(*) Note that trend 2 experiments described below, investigate the reactivity trend tert  >  sec  >  prim by keeping the halogen atom constant.

 

Trend 2 The relative reactivity of primary, secondary and tertiary halogenoalkanes

You can do a similar comparison experiment to determine the relative reactivity of isomeric (ideally) primary, secondary and tertiary halogenoalkanes, but here you keep the halogen atom constant.

Again, you make up equal concentration mixtures of the halogenoalkane, ethanol and aqueous silver nitrate.

As the halogenoalkane hydrolyses, the freed halide ion forms a precipitate with silver nitrate.

You may need to warm all the solutions to 50-60oC in a thermostated water bath to effect all three reactions.

Again, you note the time the precipitate is first clearly visible for the reaction: Ag+(aq)  +  X-(aq)  ===>  AgX(s)

The following equations involve the four isomers of the halogenoalkane general formula C4H9Br, (same for Cl or I)

For a fair comparison test, you should just vary the structures and keep the halogen atom constant.

(d) A primary halogenoalkane giving a primary alcohol on hydrolysis with water

hydrolysis conversion:      (displayed formulae change)

1-bromobutane (bpt 101oC)  +  water  ===>  butan-1-ol  +  hydrobromic acid

CH3CH2CH2CH2Br  +  H2   CH3CH2CH2CH2OH  +  H+  +  Br

The silver bromide precipitate is relatively very slow to form: Ag+(aq)  +  Br-(aq)  ===>  AgBr(s)

 

(e) Another isomeric primary halogenoalkane giving a primary alcohol on hydrolysis with water

hydrolysis conversion:     alcohols and ether structure and naming (c) doc b

1-bromo-2-methylpropane (bpt 91oC)  +  water  ===>  2-methylpropan-1-ol  +  hydrobromic acid

(CH3)2CHCH2Br  +  H2   (CH3)2CHCH2OH  +  H+  +  Br

Again, the silver bromide precipitate is relatively very slow to form: Ag+(aq)  +  Br-(aq)  ===>  AgBr(s)

 

(f) A secondary halogenoalkane giving a secondary alcohol on hydrolysis with water

hydrolysis conversion: alcohols and ether structure and naming (c) doc b   (structured formulae change)

2-bromobutane (bpt 91oC)  +  water  ===>  butan-2-ol  +  hydrobromic acid

CH3CHBrCH2CH3  +  H2   CH3CH(OH)CH2CH3  +  H+  +  Br

Here, the silver bromide precipitate forms a bit faster: Ag+(aq)  +  Br-(aq)  ===>  AgBr(s)

 

(g) A tertiary halogenoalkane giving a tertiary alcohol on hydrolysis with water

hydrolysis conversion:   alcohols and ether structure and naming (c) doc b   (structured formulae change)

2-bromo-2-methylpropane (bpt 73oC)  +  water  ===>  2-methylpropan-2-ol  +  hydrobromic acid

(CH3)3C-Br  +  H2   (CH3)3C-OH  +  H+  +  Br

Here, the silver bromide precipitate forms the faster: Ag+(aq)  +  Br-(aq)  ===>  AgBr(s)

 

You can repeat the experiment with a series of chloroalkanes or iodoalkanes (preferably isomers) and you should get the same reactivity order which is ...

Halogenoalkane reactivity trend: tertiary  >  secondary  >  primary

Explanation

Tertiary haloalkanes hydrolyse via SN1 unimolecular mechanism (see section 3.3.2), which has a lower activation energy than the SN2 mechanism by which primary haloalkanes hydrolyse (see section 3.3.4).

Secondary haloalkanes can hydrolyse via both mechanistic pathways and so are intermediate in reactivity.

relative stability of carbocations tertiary > secondary > primary > methyl advanced A level organic chemistry 

The relative ease of formation of carbocations determines the reactivity trend for the SN1 mechanism.

The more alkyl groups attached to the carbon of the C-X bond (X = halogen), the greater the electron cloud shift to stabilise the carbocation - known as the inductive effect (+I effect).

The more easily the carbocation is formed (lower activation energy), the more likely the chance of the C-X bond breaking heterolytically to form the carbocation.

Hence, with reference to the carbocation diagram above, for the same halogen X, the reactivity of the halogenoalkane is

tertiary  >  secondary  >  primary

 


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3.3.4 The mechanism of hydrolysis of primary haloalkanes by water (SN2 mechanism pathway)

SN2 bimolecular mechanism hydrolysis of halogenoalkanes haloalkane with water haloalkanes transition state advanced organic chemistry notes doc brown

mechanism 35 nucleophilic substitution of a halogenoalkane by water (SN2 bimolecular)

This is the typical SN2 mechanism by which primary halogenoalkanes hydrolyse, doesn't involve a carbocation

General scheme for the SN2 nucleophilic substitution reaction of e.g. a primary halogenoalkane undergoing direct hydrolysis with water. RCH2X where R = H or alkyl and X is the halogen atom Cl, Br or I).

Step (1): The Cδ+-Xδ- bond is polar because of the difference in electronegativity between carbon (2.5) and chlorine (3.0), so the electron rich nucleophile, the water molecule, attacks the partially positive carbon.

The nucleophilic water acts as an electron pair donor (Lewis base) to bond with the 'delta positive' carbon to give the C-O bond of the protonated alcohol.

Simultaneously the chlorine atom is ejected, taking with it the C-X bond pair, so forming the chloride ion on expulsion.

Step (1) is the rate determining step (rds) and the rate effectively only depends on the halogenoalkane concentration.

BUT the intermediate is [R3COH2]+ NOT an R3C+ carbocation.

In step (2) another water molecule rapidly accepts the proton from the protonated alcohol to leave the free alcohol product.

This mechanism is most likely with primary halogenoalkanes, but very slow if at all with water, much faster with the hydroxide ion, a more powerful nucleophile (negative ion as well as an electron pair donor).

Tertiary halogenoalkanes tend to react by the SN1 mechanism involving a carbocation, secondary halogenoalkanes react via both mechanisms.

 

The reaction progress profile for the hydrolysis of a primary halogenoalkane with water via the SN2 'transition state' mechanism - bimolecular rate determining step

 Reaction progress profile for SN2 bimolecular hydrolysis by water of halogenoalkanes advanced organic chemistry notes doc brown

Mechanism diagram 45b: Reaction progress profile for SN2 bimolecular hydrolysis by water of halogenoalkanes

Note the highest activation energy, Ea1, is by far the largest of the two and so the bimolecular step 1 is the rate determining step. The H+ product is actually a H3O+ via a 2nd water molecule.

 

SN2 bimolecular mechanism hydrolysis of bromoethane with water haloalkanes protonated alcohol advanced organic chemistry notes doc brown

Mechanism diagrams 71a and 71c show the hydrolysis of bromoethane by water.

transition state SN2 bimolecular mechanism hydrolysis of bromoethane with water haloalkanes advanced organic chemistry notes doc brown

Note the concept of the transition state in the reaction profile *, think of a C-X bond half-broken and the C-O bond half-formed at the point in the mechanism - at the top of the potential energy hump!

The H+ product is actually a H3O+ via a 2nd water molecule.

The [transition state], sometimes called the 'activation complex' is neutral because the nucleophile water is also neutral.

The [transition state] (or activated complex), is the point where the C-Br is half broken and the C-O bond is half formed.


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3.3.5 The hydrolysis of secondary halogenoalkanes

Secondary halogenoalkanes can hydrolyse by either an SN1 or SN2 mechanism.

The SN1 mechanism is faster than the SN2 pathway, and so is the dominant mechanism hence rate of hydrolysis order is tert  >  sec  >  prim

e.g. quoting from section 3.3.3 on reactivity trends

The tertiary halogenoalkane, 2-chloro-2-methylpropane, produces an almost instant white precipitate of silver chloride when mixed with ethanolic silver nitrate solution at room temperature.

This contrasts with the very slow hydrolysis of isomeric primary halogenoalkane 1-chlorobutane, that only produces a white precipitate after warmed to ~60oC for some time.

In exams you should be given equal credit for describing and SN1 or SN2 mechanism for the hydrolysis of a secondary halogenoalkane.


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3.3.6 SN1 and SN2 water hydrolysis mechanisms, rate expressions, orders of reaction

diagram structure of halogenoalkane haloalkane functional group general structural formula advanced level organic chemistry Reminder !

You must know the structures of primary, secondary and tertiary halogenoalkanes (haloalkanes)

Don't worry if you haven't done rate expressions and orders of reaction yet, you will do them later in your course.

 

SN1 mechanism hydrolysis with water tertiary halogenoalkane haloalkane carbocation intermediate advanced organic chemistry notes doc brown

Reminder of what we mean by an SN1 nucleophilic substitution reaction (e.g. unimolecular mechanism 10 above).

The S signifies substitution, N signifies nucleophilic (attack) and the 1 means a unimolecular step 1 that also determines the rate of the reaction because the heterolytic bond fission of the C-Cl bond is the slowest rate determining step and only one molecule involved - hence the phrase 'unimolecular'.

The rate of the reaction is controlled ONLY by the concentration of the haloalkane for a given solvent:

rate = k1[RX]

k1 = 1st order rate constant, [RX] = concentration of haloalkane in the rate expression.

The concentration of water is irrelevant to the rate of the reaction.

If the hydrolysis kinetics show up as a 1st order rate expression, it is indicative of a SN1 unimolecular carbocation mechanism.

 

SN2 bimolecular mechanism hydrolysis of halogenoalkanes haloalkane with water haloalkanes transition state advanced organic chemistry notes doc brown 

Reminder of what an SN2 nucleophilic substitution reaction is.

S signifies substitution, N signifies nucleophilic and the 2 means a bimolecular step1 that also determines the rate of the reaction via a transition state - known as the rate determining step by this particular mechanism.

Don't worry if you haven't done rate expressions and orders of reaction yet, you can ignore the next paragraph until later in your course.

The rate of the reaction is controlled by the concentration of the haloalkane and the water:

rate = k2[RX][H2O], k2 = 2nd order rate constant, [RX] = concentration of haloalkane,

and [H2O] = concentration of water, in the rate expression.

Since water or aqueous ethanol is the solvent, the concentration of water is so high, it is effectively constant, so the concentration of the halogenoalkane controls the rate.

The kinetics simplifies to: rate = k[RX]

However, this is not the case for the hydrolysis using sodium hydroxide solution (Part 3.4).

The term 'bimolecular' in this case refers to the two molecule collision in the rate determining step 1  - between the halogenoalkane and the water molecule.

This is deduced from the equation, not from experiments.

The order of the reaction, is the sum of the powers to which the concentration terms are raised in the rate expression (e.g. here it is 1 + 1 = 2 = 2nd order rate expression).

If the hydrolysis kinetics show up as a 2nd order rate expression, it is indicative of a SN2 bimolecular mechanism.

 

The order of reaction can only be determined by experiment - see

Obtaining rate data, interpreting data, deducing orders of reaction and rate expressions

 


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