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Chapter 4
Alcohols and Alkyl Halides
Functional Groups
A functional group is a structural unit in a molecule
responsible for its characteristic physical properties as
well as its behavior under a particular set of reaction
conditions.
Alkenes and alkynes are examples of functional groups.
In this chapter we specifically meet alkylhalides and
alcohols
Nomenclature
IUPAC rules permit the use of two different naming
conventions. One is functional class nomenclature the
other is substitutive nomenclature.
Substitutive nomenclature is preferred.
Functional class nomenclature is more common.
Functional Class Nomenclature
of Alkyl Halides
The alkyl group and the halide are listed separately in
the name. The alkyl group is the longest chain starting
at the carbon that has the halogen attached. Other
alkyl groups are listed as substituents.
Substitutive Nomenclature
of Alkyl Halides
Alkyl halides have a halo (fluoro-, chloro-, bromo- and
iodo-) substituents on an alkane chain.
The halogen is treated as a substituent.
The carbon chain is numbered from the side closest
the substituent as before.
Substitutive Nomenclature
of Alkyl Halides
Number from side closest to substituent. The halogen
and alkyl groups have the same priority.
In the name list the substituent alphabetically.
2-chloro-5-methylheptane
5-chloro-2-methylheptane
IUPAC Nomenclature of Alcohols
Functional class names have the alkyl name followed by
alcohol as a separate word.
Substitutive names start with the longest contiguous
carbon chain that bears the –OH group. Number from the
side closest to the OH group and replace the –ane of the
corresponding alkane with –ol.
List substituents and their locants before the parent name.
IUPAC Nomenclature of Alcohols
The OH is assumed to be attached to C-1 of cyclic alcohols.
3-ethylcyclopentanol
4-bromobutan-1-ol
Classes of Alcohols and Alkyl Halides
Classification of Alkyl Halides
Alkyl halides are defined as primary if the carbon that the
halogen is attached to is directly attached to one other
carbon. Similarly if the carbon that the halogen is attached
to is directly attached to two carbons then it is a secondary
alkyl halide. In tertiary alkyl halides the carbon with the
halogen attached is directly attached to three other carbons.
secondary
tertiary
primary
Classification of Alcohols
Alcohols are defined as primary, secondary or tertiary in the
same way. If one, two or three other carbons are directly
attached to the carbon that the OH is attached to then the
alcohol is primary, secondary or tertiary respectively.
tertiary
secondary
primary
Bonding in Alcohols and Alkyl Halides
Bonding in Alcohols
The C-O bond is made by overlap of an sp3 orbital on carbon
with one on oxygen. The oxygen has two non bonding
electron pairs.
Bonding in Alkyl Halides
The halogen is connected to the carbon with a s bond.
The carbon-halogen bond distances increase in the order:
C-F < C-Cl < C-Br < C-I
Alkyl halides and alcohols have polar bonds and may be
polar molecules.
Physical Properties of Alcohols and Alkyl
Halides: Intermolecular Forces
Dipole-dipole Attractive Force
Molecules with permanent dipoles have a stronger dipoledipole intermolecular interaction than alkanes.
Consequently fluoroethane has a higher boiling point than
propane despite being almost the same size.
Dipole-dipole interactions are not enough to explain the
exceptionally high boiling point of ethanol.
Alcohols and Hydrogen Bonding
Alcohols have a special type of dipole-dipole interaction
called hydrogen bonding. The partially positive proton of one
⎯ OH group interacts with the partially negatively oxygen of a
second ethanol.
The oxygen is termed the hydrogen bond acceptor and the
OH hydrogen the hydrogen bond donor.
Alcohols and Hydrogen Bonding
Ball and stick model showing a hydrogen bond between two
ethanol molecules.
Alcohols and Hydrogen Bonding
A space-filling model showing the electrostatic potential for
two hydrogen bonded ethanol molecules.
Hydrogen Bonding
Hydrogen bonds are 15-20 times weaker than covalent
bonds.
Hydrogen bonding in organic compounds involves O and N
only:
Hydrogen bonds are strong enough to impose structural
order on many systems.
Boiling Points
Iodine is highly polarizable because the valence electrons
are far from the nucleus. Therefore the induced dipoleinduced dipole attractive forces dominate.
Boiling Points
Increasing the number of halogens (Cl, Br or I) also
increases the induced dipole-induced dipole attractive
forces and therefore also the boiling point.
Boiling Points
Fluorine has very low polarizability and the boiling points
do not increase with increasing numbers of fluorine atoms.
Solubility in Water
Alkyl halides are insoluble in water whereas the solubilty
of alcohols in water is directly related to the size of the
alkyl group the OH is attached to.
Methyl, ethyl, n-propyl, and isopropyl alcohols are all
totally miscible in water (soluble in all proportions) but
only 1 mL 1-octanol dissolves in 2000 mL of water.
Hydrogen bonding between
ethanol and water.
Density
Alkyl fluorides and chlorides are less dense, and alkyl
bromides and iodides more dense, than water.
Increasing halogenation increases density so CH2Cl2 is
more dense than water.
Preparation of Alkyl Halides from
Alcohols and Hydrogen Halides
Preparation of Alkyl Halides
Synthesis.
The rest of this chapter focusses on methods of
preparation of alkyl halides.
Mechanism.
The step-by-step description of how reactions take
place will be introduced.
Preparation of Alkyl Halides
Reaction of alcohols with hydrogen halides yields
alkyl halides:
Reactivity of the alcohols is directly related to the nature
of the alcohol:
Rate of Reaction
Tertiary alcohols react fastest at low temperature and
primary slowest needing higher temperatures:
Reaction of Alcohols with Hydrogen
Halides: The SN1 Mechanism
The Reaction Equation
The reaction equation describes the overall process
from reactants on the left to products on the right.
The mechanism will show how this reaction occurs.
The Reaction Mechanism: Step 1
The reaction mechanism is the step-by-step pathway
describing how the reaction takes place.
Step 1: Protonation
The alcohol acts as Brønsted base and is protonated by
the strong acid. Chloride is the conjugate base of HCl.
This is a bimolecular reaction – both reactants change.
The tert-butyloxonium cation is an intermediate.
Proton Transfer
The change in energy of Step 1 can be plotted on a
potential energy diagram.
The transition state is not a stable structure and the bonds
are partially formed and partially broken at this point.
The activation energy is low and the step is exothermic.
The Reaction Mechanism: Step 2
Step 2: Dissociation.
The second step is unimolecular and results in the
formation of an intermediate carbocation.
Carbocation Formation
The carbocation intermediate is a relatively unstable
species and is therefore high energy (the central carbon
does not have an octet of electrons).
Overall step 2
is endothermic.
Carbocations
The central carbon of the
carbocation is sp2 hybridized.
The positive charge is in the
empty p-orbital.
Carbocations are electrophilic
(electron seeking or electron
loving) and are Lewis Acids.
The Reaction Mechanism: Step 3
The last step is a Lewis acid-Lewis base reaction. The
chloride is called a nucleophile (nucleus seeker). This
is a bimolecular reaction.
Step 3: Chloride attaches to the carbocation.
Reaction of t-Butyl Cation with Chloride
An unstable intermediate reacts to form stable products
very exothermic.
A very favorable process so the activation energy is low.
Reaction of t-Butyl Cation with Chloride
The nucleophile has a nonbonding electron pair in a p-orbital
that interacts with the empty p-orbital of the carbocation
to form a s-bond.
The Reaction Mechanism
The sum of each individual step in the reaction
mechanism must equal the overall reaction equation.
The reaction is a substitution reaction in which the
nucleophile chloride takes the place of the OH. Thus,
it is known as an SN reaction.
The slow step in the unimolecular reaction step 2 and
this is known as the rate determining step. The overall
reaction cannot go faster than this step. Thus, the
reaction is a SN1 reaction.
Confirming the Mechanism
The stereochemistry of a reaction is used to probe the
formation of a carbocation. For example, these two
isomeric alcohols should give the same carbocation:
Therefore they should form exactly the same product/s
as they do – a 4:1 mixture of isomers.
Structure, Bonding,
and Stability of Carbocations
Stability of Cations
Alkyl groups directly attached to the positively charged
carbon stabilize a carbocation.
Carbocations are defined as primary, secondary or tertiary
depending on how many carbons are directly attached to
the cationic carbon.
Stability of Cations
The stability of cations can be modeled and the spread
of the positive charge (blue/violet color) seen in the
electrostatic potential maps.
Methyl cation has intense positive charge. The more the
charge is delocalized the more stable the cation is.
Stability of Cations
A carbocation is stabilized by delocalization
of electrons from s-bonds b to the positively
charged carbon into the empty p-orbital.
The valence bond model shows orbital overlap.
MO theory predicts a bonding orbital with 2 electrons that
spans the b s-bond and the positive carbon.
Effect of Alcohol Structure on
Reaction Rate
The Alcohol Carbocation Connection
The rate determining step is:
The rate is only proportional to the
concentration of the alkyloxonium
cation.
The Alcohol-Carbocation Connection
The rate of formation of the carbocation is related to the
stability of the carbocation formed. The transition state
is closer in energy to the carbocation so the activation
energy tracks the stability of the carbocation.
Reaction of Methyl and Primary Alcohols
with Hydrogen Halides:
The SN2 Mechanism
Reaction Equation
The overall reaction equation looks the same as that for
tertiary alcohols: a substitution reaction.
Mechanism Step 1
The first step of the reaction is the same.
Step 1: Proton transfer.
The second step of the reaction cannot be the same
because the primary carbocation is too unstable
Mechanism Step 2
Since primary and methyl carbocations are very unstable
primary alcohols must react in a different way.
Step 2. Nucleophilic displacement.
Step 2 is the slower step and therefore is the rate
determining step. Since this is a bimolecular reaction
it is given the symbol SN2.
Other Methods for Converting
Alcohols to Alkyl Halides
Reaction of Alcohols with Thionyl Chloride
Thionyl chloride is mainly used to transform primary and
secondary alcohols into alkyl chlorides.
Pyridine is a base used to neutralize the acid (HCl) formed.
The Thionyl Chloride Reaction Mechanism
The key step in the reaction is an SN2 type of reaction:
Reaction of Alcohols with
Phosphorous Tribromide
Phosphorous tribromide (PBr3) reacts with alcohols to
form alkyl bromides. This is also an SN2 reaction.
Halogenation of Alkanes
Overall reaction equation:
RH + X2  RX + HX (X=F, Cl, Br or I)
For F2 the reaction is explosive
For I2 the reaction is endothermic and not feasible
The reaction is exothermic and useful for Cl2
and Br2
Chlorination of Methane
An industrially important reaction carried out at high
temperature (400 oC). Four products formed sequentially.
CH4 + Cl2  CH3Cl + HCl
CH3Cl + Cl2  CH2Cl2 + HCl
CH2Cl2 + Cl2  CHCl3 + HCl
CHCl3 + Cl2  CCl4 + HCl
The reaction has free radicals as intermediates.
Structure and Stability of Free
Radicals
Free Radicals
Free radicals contain unpaired electrons. Some common
free radicals are:
Free Radicals
Free radicals with the unpaired electron on a carbon are
defined as primary, secondary or tertiary depending on
how many carbons are directly attached to the C with the
unpaired electron.
Free Radicals
The carbon atom with the unpaired electron is best
described as sp2 hybridized.
Stability of Free Radicals
The spin density (shown in yellow) is shared onto
connected alkyl groups thereby stabilizing the radical .
More substituted radicals are more stable.
Stability of Free Radicals
More substituted radicals are more stable.
Bond Cleavage
In homolytic bond cleavage each atom retains one of the
bonding electrons. The energy required is the bond
dissociation enthalpy (D).
In heterolytic cleavage the more electronegative element
retains both bonding electrons.
homolytic
heterolytic
Bond Dissociation Enthalpies
Comparing the energy required for homolytic bond cleavage
provides a way to quantify radical stability. Compare these
two reactions:
The only difference is the type of radical formed and the
energy released.
Bond Dissociation Enthalpies
Potential energy graph of the two homolytic cleavage
reactions. This provides information on the stability of
radicals.
Less energy:
more stable
radical.
Free Radical Chlorination of Methane
We will consider monochlorination.
The mechanism has three parts: initiation, propagation,
and termination.
Chlorination of Methane: Initiation
Step 1: Dissociation.
The chlorine-chlorine bond is broken homolytically.
This bond is the weakest in the reaction mixture.
Heat or light can provide the necessary energy.
Arrows with a single “hook” are used to show the movement
of a single electron.
Chlorination of Methane: Propagation
Step 2: Hydrogen atom abstraction.
A chlorine radical abstracts a hydrogen atom from a
methane molecule forming HCl and a methyl radical.
The methyl radical reacts in the next step (it propagates the
reaction).
Chlorination of Methane: Propagation
Step 3: Chlorine atom abstraction.
The methyl radical abstracts a chlorine atom from a chlorine
molecule to form chloromethane and another chlorine radical.
The chlorine radical starts another propagation cycle – step 2.
Steps 2 and 3 form a radical chain reaction.
Few radicals are needed to initiate the chlorination of methane.
Chlorination of Methane: Termination
When two radicals react they effectively stop the propagation
steps and are therefore known as chain-termination steps.
Examples are:
Halogenation of Higher Alkanes
Reaction in labs use light (hu) to initiate the reaction.
Cyclobutane yields a single monochlorination product
since abstraction of any of the 8 H atoms results in the
same product being formed.
In contrast, butane yields a 28:72 mixture of 1chlorobutane to 2-chlorobutane.
Selectivity of Halogenation
There are 6 primary hydrogens and 4 secondary
hydrogens in butane.
Therefore the expected ratio is 60:40.
The 28:72 ratio therefore means that 2-chlorobutane
is preferentially formed. Why?
Selectivity of Halogenation
There must be selectivity when the hydrogen atom is
abstracted. The preferred reaction is:
instead of:
Selectivity of Halogenation
The transition state is lower in energy for abstraction
of a secondary hydrogen because a secondary radical
is more stable:
The relative rates of hydrogen abstraction are:
Selectivity of Chlorination
Based on these experiments the relative rates of
chlorination are determined:
Selectivity of Bromination
The relative rates of bromination are :
Bromination is highly selective favoring tertiary substitution:
Chlorination vs Bromination
The hydrogen atom abstraction is exothermic for
chlorination and endothermic and therefore more
selective for bromination: