Chapter 4 Alcohols and Alkyl Halides

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Chapter 4Alcohols and Alkyl Halides
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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

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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.

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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.

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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.
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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
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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.

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IUPAC Nomenclature of Alcohols

• The OH is assumed to be attached to C-1 of cyclic
  alcohols.

3-ethylcyclopentanol
4-bromobutan-1-ol
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Classes of Alcohols and Alkyl Halides
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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
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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.

secondary
tertiary
primary
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Bonding in Alcohols and Alkyl Halides
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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.

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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 lt C-Cl lt
  C-Br lt C-I
• Alkyl halides and alcohols have polar bonds and
  may be polar molecules.

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Physical Properties of Alcohols and
AlkylHalides Intermolecular Forces
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Dipole-dipole Attractive Force

• Molecules with permanent dipoles have a stronger
  dipole-dipole 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.
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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.
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Alcohols and Hydrogen Bonding

• Ball and stick model showing a hydrogen bond
  between two ethanol molecules.

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Alcohols and Hydrogen Bonding

• A space-filling model showing the electrostatic
  potential for two hydrogen bonded ethanol
  molecules.

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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.
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Boiling Points
Iodine is highly polarizable because the valence
electrons are far from the nucleus. Therefore
the induced dipole-induced dipole attractive
forces dominate.
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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.
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Boiling Points
Fluorine has very low polarizability and the
boiling points do not increase with increasing
numbers of fluorine atoms.
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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.
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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.
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Preparation of Alkyl Halides fromAlcohols and
Hydrogen Halides
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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.

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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
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Rate of Reaction

• Tertiary alcohols react fastest at low
  temperature and primary slowest needing higher
  temperatures

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Reaction of Alcohols with HydrogenHalides The
SN1 Mechanism
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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.
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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.
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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.
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The Reaction Mechanism Step 2
Step 2 Dissociation.

• The second step is unimolecular and results in
  the formation of an intermediate carbocation.

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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.

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Carbocations

• The central carbon of the carbocation is sp2
  hybridized.The positive charge is in theempty
  p-orbital.
• Carbocations are electrophilic(electron seeking
  or electronloving) and are Lewis Acids.

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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.
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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.

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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.

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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
  thenucleophile 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.

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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 41 mixture of isomers.
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Structure, Bonding, and Stability of
Carbocations
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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.
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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.
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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.

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Effect of Alcohol Structure on Reaction Rate
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The Alcohol Carbocation Connection

• The rate determining step is

The rate is only proportional to the
concentration of the alkyloxonium cation.
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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.

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Reaction of Methyl and Primary Alcohols with
Hydrogen Halides The SN2 Mechanism
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Reaction Equation

• The overall reaction equation looks the same as
  that for
• tertiary alcohols a substitution reaction.

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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
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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 reactionit is given the symbol SN2.
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Other Methods for Converting Alcohols to Alkyl
Halides
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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.
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The Thionyl Chloride Reaction Mechanism

• The key step in the reaction is an SN2 type of
  reaction

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Reaction of Alcohols with Phosphorous Tribromide

• Phosphorous tribromide (PBr3) reacts with
  alcohols to
• form alkyl bromides. This is also an SN2 reaction.

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Halogenation of Alkanes

• Overall reaction equation
• RH X2 ? RX HX (XF, 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

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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.

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Structure and Stability of Free Radicals
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Free Radicals

• Free radicals contain unpaired electrons. Some
  common free radicals are

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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.
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Free Radicals
The carbon atom with the unpaired electron is
best described as sp2 hybridized.
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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.
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Stability of Free Radicals
More substituted radicals are more stable.
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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
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Bond Dissociation Enthalpies
Comparing the energy required for homolytic bond
cleavageprovides a way to quantify radical
stability. Compare these two reactions
The only difference is the type of radical formed
and the energy released.
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Bond Dissociation Enthalpies
Potential energy graph of the two homolytic
cleavage reactions. This provides information
on the stability of radicals.
Less energymore stableradical.
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Free Radical Chlorination of Methane
We will consider monochlorination.
The mechanism has three parts initiation,
propagation, and termination.
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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 movementof a single
electron.
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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).
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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.
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Chlorination of Methane Termination
When two radicals react they effectively stop the
propagation steps and are therefore known as
chain-termination steps.Examples are
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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 2872 mixture of
1-chlorobutane to 2-chlorobutane.
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Selectivity of Halogenation
There are 6 primary hydrogens and 4 secondary
hydrogens in butane. Therefore the expected
ratio is 6040. The 2872 ratio therefore means
that 2-chlorobutaneis preferentially formed.
Why?
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Selectivity of Halogenation
There must be selectivity when the hydrogen atom
is abstracted. The preferred reaction
is instead of
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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
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Selectivity of Chlorination
Based on these experiments the relative rates of
chlorination are determined
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Selectivity of Bromination
The relative rates of bromination are
Bromination is highly selective favoring tertiary
substitution
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Chlorination vs Bromination
The hydrogen atom abstraction is exothermic for
chlorination and endothermic and therefore more
selective for bromination