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Foundation Science Lecture 4


									Foundation Science Lecture 4

 CHM403 Foundation Chemistry II
    Alkyl Halides & Amines
       Alkyl Halides and Nucleophilic Substitution
Introduction to Alkyl Halides

• Alkyl halides are organic molecules containing a halogen atom bonded to an sp3
  hybridized carbon atom.
• Alkyl halides are classified as primary (1°), secondary (2°), or tertiary (3°),
  depending on the number of carbons bonded to the carbon with the halogen
• The halogen atom in halides is often denoted by the symbol “X”.


   Common names are often used for simple alkyl halides. To assign a common
       Name all the carbon atoms of the molecule as a single alkyl group.
       Name the halogen bonded to the alkyl group.
       Combine the names of the alkyl group and halide, separating the words
       with a space.

Physical Properties

• Alkyl halides are weak polar molecules. They exhibit dipole-dipole interactions
  because of their polar C—X bond, but because the rest of the molecule
  contains only C—C and C—H bonds, they are incapable of intermolecular
  hydrogen bonding.

The Polar Carbon-Halogen Bond

 The pattern in strengths of the four carbon-halogen bonds are:

    Notice that bond strength falls as you go from C-F to C-I, and notice how much stronger the
    carbon-fluorine bond is than the rest.
The Polar Carbon-Halogen Bond

• Effect of Bond Polarity
• Of the four halogens, fluorine is the most electronegative and iodine the
  least. That means that the electron pair in the carbon-fluorine bond will be
  dragged most towards the halogen end.
• The electronegativities of carbon and iodine are equal and so there will be
  no separation of charge on the bond.

Reactions of Alkyl Halides

General Features of Nucleophilic Substitution

• Three components are necessary in any substitution reaction.

     The Nucleophile

• Nucleophiles and bases are structurally similar: both have a lone pair or a  bond. They differ in what
  they attack.

Reaction of Alkyl Halides with Hydroxide ions

 Substitution or elimination?
 There are two different sorts of reaction that you can get depending on the conditions used and
 the type of halogenoalkane. Primary, secondary and tertiary halogenoalkanes behave differently
 in this respect.

     Substitution reactions
     In a substitution reaction, the halogen atom is replaced by an -OH group to give an alcohol.
     For example:

                      Or, as an ionic equation:                   Reaction Mechanism- To be
                                                                  discussed in lecture
Elimination reactions

Halogenoalkanes also undergo elimination reactions in the presence:
of sodium or potassium hydroxide.

    Reaction Mechanism- To be discussed in
Reaction with Ammonia

The reaction of ammonia with an alkyl halide leads to the formation of a primary amine. The
primary amine that is formed can also react with the alkyl halide, which leads to a disubstituted
amine that can further react to form a trisubstituted amine.
What decides whether you get substitution or elimination?

 The type of halogenoalkane
 This is the most important factor.

       type of halogenoalkane            substitution or elimination?
                 primary                        mainly substitution
                secondary                both substitution and elimination
                 tertiary                       mainly elimination

   The solvent                                         The temperature
   The proportion of water to ethanol in the solvent   Higher temperatures encourage
   matters.                                            elimination.
   Water encourages substitution.                      Higher concentrations favour
   Ethanol encourages elimination.                     elimination.
Reaction with Ethoxide ions (alkoxide ions)

                       Essentially, ethoxide (and other alkoxide) ions behave
                       like hydroxide ions.

  Ethoxide ions are strongly basic
  If you add water to sodium ethoxide, it dissolves to give a colourless solution with a high pH -
  typically pH 14. The solution is strongly alkaline.

   Ethoxide ions are good nucleophiles
   A nucleophile is something which carries a negative or partial negative charge which it uses to
   attack positive centres in other molecules or ions.

                                Bases attack protons    !!!!!!

                                Refer to Hydroxide ion reactions!!!!!!!!!!!!
Organic Chemistry


 Amines are organic compounds in which one or
 more H in ammonia, NH3, is replaced with alkyl or
 aromatic groups.

  H               H             H
  |               |             |
H—N—H         CH3—N—H       CH3—N—CH3
   Classification of Amines
  Amines are classified as primary, secondary, or tertiary.
 In a primary (1°) amine, one carbon group is bonded to
  the nitrogen atom.
 A secondary (2°) amine has two carbon groups.
 A tertiary (3°) amine has three carbon groups.
        H                 CH3                  CH3
        |                   |                  |
  CH3—N—H            CH3—N—H            CH3—N—CH3
         1°                  2°                3°
  IUPAC Names of Amines
 In the IUPAC system, amines are named as
 The –e in the alkane name of the longest chain is
  changed to –amine.
 The chain is numbered to locate the amine group
  and substituents.
  CH3—CH2—NH2 CH3—CH—CH3
  Ethanamine                2-Propanamine
   Boiling Points of Amines, Alcohols, and Alkanes
The boiling points of amines are higher than alkanes,
but lower than alcohols of similar mass.
   Hydrogen Bonding for Amines
 The polar N-H bond provides hydrogen bonding in
  1°and 2° amines, but not 3°.
 However, the N-H bonds in amines are not as polar as
  the O-H bonds in alcohols.
   Solubility in Water
 Amines with 1-5 carbon atoms are soluble in water.
 The N atom in amines forms hydrogen bonds with
 the polar O-H bond in water.
   Preparation of Amines
 Alkylation of ammonia
   ammonia reacts with an alkyl halide to form primary amine.
   primary amine reacts with the alkyl halide again to form secondary amine
   Secondary amine reacts with alkyl halide again to for tertiary amine
    Preparation of Amines
 Reduction of Amides
   Amides yield primary amines on reduction by lithium aluminum hydride, while N-
     substituted and N, N-disubstituted amides produce secondary and tertiary amines,

NB: Because amides are easily prepared, their reduction is a preferred method for making all classes
of amines.
Reaction of Ammines
 Acid halides react with amines to form substituted amides.

Note: The second product formed is HCl which reacts with excess amine to form a salt.
  Reaction of Ammines
 Acid Anhydrides
   Similar reaction as for acid halides

 Note: The second product formed is Ethanoic acid which reacts with excess amine to form a
 salt, but this time ethanoate.
   Amines React as Bases
 Like ammonia, amines are weak bases in water.
 NH3 + H2O                    NH4+ + OH–

 CH3—NH2 + H2O                CH3—NH3+ + OH–
    Properties as bases
 Like ammonia, amines are bases. Amines are stronger bases than ammonia.

        Ions of compound         Kb                 pKb
        (CH3)2NH                 5.4·10−4 M         3.26
        CH3CH2CH2NH2             4.7·10−4 M         3.32
        CH3NH2                   4.4·10−4 M         3.35
        (CH3)3N                  5.9·10−5 M         4.42
        (CH3)2CHNH2              3.4·10−4 M         3.46
        NH3                      1.8·10−5 M         4.74

 The larger the pKb , the weaker the base.
   Neutralization forms Amine Salts
An amine salt:
 Forms when an amine is
  neutralized by acid.
 Is named by replacing the
  amine part of the name with
  ammonium followed by the
  name of the negative ion.
CH3—NH2 + HCl             CH3—NH3+ Cl–
  Methylamine            methylammonium chloride
   Properties of Amine Salts
Amine salts are:
 Solids at room temperature.
 Soluble in water and body fluids.
 The form used for drugs.
The End

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