Chapter 4, Alcohols and Alkyl Halides

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					                     Chapter 4, Alcohols and Alkyl Halids                                  1

Functional groups: Organic molecules can undergo a wide variety of chemical reactions,
only certain portions of their structures are changed in any particular reaction, this portion
is called functional group.

Alcohols: The functional group of an alcohol is an- OH (Hydroxyl group) bonded to a
carbon.
Alkyl halids: The functional group of alkyl halids is a Halogen, F,Cl, Br, I.
Naming of Alcohols :
1. Choose the longest chain as the parent alkane.
2. Change the ending of the parent alkane from e to ol.
3. Carbon number one is the closest carbon to OH group even there are alkyl or halogen
    branches..
4. Name and number substituents, and list them in alphabetical order.



                                                      OH

                OH
   2-methyl-3-pentanol                          2-Butanol


                OH


Cyclopentanol                 FCH2CH2CH2OH 3-Fluoro-1-propanol


Physical properties of alcohols: Alcohols have one polar part O –H and one nonpolar
part (hyrocarbon part), all physical properties of alcohols depend on the ratio of these two
parts.
Boiling point of a substance depends on intermolecular forces between molecules of the
substance. Hydrogen bonding, dipole-dipole and london dispersion are intermolecular
forces between molecules in an alcohol.
Hydrogen bonding and dipole-dipole interaction decrease with increasing hydrocarbon
part in alcohols but london dispersion forces increase with increasing of molecular
weight, which means heavier alcohols have more london dispersion and less polarity.

Solubility in water: Polar alcohols are soluble in water. Polarity of alcohol decreases
with increasing of hydrocarbon part, so alcohol becomes less soluble in water.




             Chem. 261 Bellevue College M - Heydari
                     Chapter 4, Alcohols and Alkyl Halids                                2

Naming of Alkylhalids:
1. The name of the halogen is changed to fluoro,chloro, bromo or iodo.
2. Carbon number one is closest to halogen.
3. Halogen and alkyl branch on the chain are considered of equal rank. So number the
   chain in order to get lower number to the substituent.




                                    Br                          Br
               F




1-fluoropentane           2-bromopentane          2-bromo-3-methylpentane




          Cl



3-chloro-2-methylpentane

Physical properties of alkylhalids:
London dispersion forces and dipole-dipole are intermolecular forces in alkylhalids.
All alkyl halides are insoluble in water. The boiling point of alkylhalides increases with
increasing number of halogens. Fluorine is an exception, increasing number of fluorines
does not lead to higher boiling point.


Structure and Reactivity in Alcohols:
O-H bond is a polar covalent bond. Alcohols can act as a base, because of two lone pairs
on oxygen, so they can react with an acid.

ROH + H-X                  R-X + H2O
Reactin Mechanism:

Each chemical reaction is involved with breaking and forming bonds. The mechanism
shows the sequence in which bonds are broken and formed, as well as energy changes
associated with each step. The reaction of an alcohol with hydrogen halide is a
substituted reaction, OH group is replaced with a halogen.




               Chem. 261 Bellevue College M - Heydari
                     Chapter 4, Alcohols and Alkyl Halids                                3

Preparation of Alkyl halides:
1. Substitution reactions  R-OH + H-X                         R-X + H2O


A. Nucleophilic Substitution First order (SN1)


1. proton Transfer

R-O-H +      H- Cl                        R - O+- H + Cl-

2. Carbocation formation: R+ is an intermediate in SN1 reaction. This step is slowest
step in the mechanism, which means the rate of reaction depends on rate of carbocation
formation.

R – O+ - H                    R+ + H2O

3. Reaction of carbocation with Anion and formation of an alkyl halide:

R+ + Cl-                RCl

Carbocations (R +) are lewis acids. They strongly are seeking for electron.
They are called electrophiles.

Negatively charged anions like Cl- are lewis bases, they have available electrons and they
strongly are seeking for electrophiles. They are called nuclephiles.

In this reaction finally an OH group is replaced with an halogen (Cl) through a
nucleophilic attack., the reaction is called nucleophilic substitution.

Because of the rate of the reaction depends on the concenteration of alcohol, this reaction
is called SN1, substitution nucleophilic,unimolecular.

Reactivity of H-X:
H-X acts as an acid in reaction with an alcohol. Reactivity of H-X increasing with
increasing acidity.
Therefore: HI  HBr  HCl  HF


Reactivity of alcohols:
Tert-alcohols  secondry alcohols  primary alcohols  methyl alcohol
Tert-alcohols react with H-Cl at room temperature very fast. Secondry and primary
alcohols donot react with H-Cl at room temperature, they react with H-Br, when
temperature increased to 1000C.




             Chem. 261 Bellevue College M - Heydari
                    Chapter 4, Alcohols and Alkyl Halids                                   4

Stability of carbocations:
Evidence shows stability of carbocations increase with increasing of alkyl substituts on
positive carbon.

                              H                                      H
                                                        H
                                                                 H       H
                                                   H


Positive carbon has electron deficiency, alkyl groups are considered as electron donors ,
so stability of carbocation increases with more electron donors groups on positive carbon.

B. Nucleophilic Substitution Second order (SN2)
How ever primary alcohols and methyl alcohols are coverted to alkyl halids. The rate of
the reaction depends on concentration of two reactants and there is no evidence of
forming of carbocations as an intermediate. The reaction is called SN2.
Sugusted mechanism:

1) R-CH2- OH + H- Cl                        R-CH2 - OH2 + Cl-

2) R-CH2 - OH2 + Cl-                     R- CH2 -Cl + H2O


2. Using Inorganic Reagents:
These methods are used mainly to covert primary and secondry alcohols to alkyl halids.

A. Using Thionyl chloride ( SOCl2)as a reagent:
                   Base
R-OH + SOCl2                 R-Cl + SO2 + HCl

B. Using phosphorus tribromide (PBr3) :

R-OH + PBr3                         R-Br    + H3PO4

                                             heat or light
3.Halogenation of Alkanes: R-H + X2                          R-X + H-X

Free Radical Mechanism:

1. Cl - Cl                        2Cl.

2. Cl.   +   H – CH3                     H-Cl + .CH3

3. Cl-Cl + .CH3                      CH3Cl + HCl




             Chem. 261 Bellevue College M - Heydari
                     Chapter 4, Alcohols and Alkyl Halids                                 5


Stability of Free Radicals:

Free radicals, like carbocations need electron, so they are stabilized by substituents, such
as alkyl groups, that release electrons. Tert-radicals are most stable free radicals.


  •                                                  •                       H
                         •                       H       H                   •
                              H                                          H       H


Bond Dissociation Energy (BDE) :

 Homolytic bond cleavage            R-H                         R +H

Heterolytic bond cleavage:        X-Y                        X+ + :Y-

The energy required for homolytic bond cleavage is called bond dissociation energy.
More stable radicals have less BDE.



      H

                                         •

                                                                 H = +380 KJ


            H

                                             •
                                                                   H = + 410 KJ


Relative Rate (chlorination):           R3CH                R2CH2      RCH3
                                         Tertiary            Secondry    Primary
                                           5.2                3.9           1
Relative Rate (bromination)              1640                 82             1




            Chem. 261 Bellevue College M - Heydari

				
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