CHAPTER 7

Haloalkanes, Arenes and Arenes

A. Structure-Alkyl group + Halogen

1. Classification-based on the carbon
atom to which the halogen atom is attached.
a. Primary (1^o) carbon=a carbon
atom which is bonded to ONE other carbon atom.
b. Secondary (2^o) carbon= a
carbon which is bonded to TWO
other carbon atoms.
c. Tertiary (3^o) carbon= a carbon
atom which is bonded to THREE
other carbon atoms.

2. Nomenclature-IUPAC rules-similar to alkanes

a. prefixes: fluoro, chloro, bromo, iodo
b. Listed in alphabetical order with other groups
c. Double bond more important than halogen

B.Physical Properties of Alkyl Halides

1. The C-X bond is polar its relative polarity is determined by the electronegativity of the halogen and the length of the C-X bond.

2. Boiling Points-similar to alkanes based on molar mass and branching of C chain.

a. Haloalkanes of the same molar mass as alkanes generally have higher boiling points.

b. Fluorides usually have LOWER boiling points than alkanes of similar molecular masses. Due to small size of F and its low polarizability.

3. Density-the density of haloalkanes is greater than hydrocarbons of similar molecular mass. They are also generally insoluble in water.

C. Halogenation-a Substitution Reaction

General Equation:



1. Monohalogenation versus Poly Halogenation




For monohalogenation use an excess of the alkane

For complete halogenation use an excess of the halogen

2. In monohalogenation each different type of hydrogen will be replaced by a halogen and result in the formation of a different alkyl halide.


3. Selectivity of Bromine-bromine replaces hydrogens in the order of preference: 3^o>2^o>1^o.





4. Mechanism of halogen-chain reaction-Three step process-initiation, propagation,termination.

Must supply energy, hn or heat





Examples:




5. The radical is any chemical species that contains one or more unpaired electron--Use “fishhook” arrows to show their formation.

6. Alkyl Free radicals

a. Structure=sp2-Planar, unshared electron in the p orbital.
b. Stability=3^o>2^o>1^o

The slow step in this process is the formation of the alkyl free radical. Tertiary free radicals have the lowest Ea and are thus formed more easily.


c. Synthesis of alkyl halides-This
is a useful synthesis of alkyl halides if all the hydrogen atoms in the alkane or cycloalkane are
all the same.




_{7. If a chiral center is generated in the reaction, the racemic modification of the compound results.}

8. Allylic Halogenation-The carbon adjacent to the C=C in an alkene is called an allylic carbon, and any attached H’s are called allylic hydrogens.

a. The allylic hydrogens have a lower bond breaking energy than 3^oH’s or vinyl H’s. (p. 250)

b. The allylic free radical is the most stable of those discussed in Chapter 7

c. Allylic bromination is accomplished by using high temperatures for bromination or a special reagent called NBS (N-bromosuccinimide in CCl₄ solvent). Under conditions of room temperature the bromine simply adds to the C=C.


d. High temperature favors allylic substitution, low temperature favors addition to p bond.

The difference in result is due to a different mechanism operating to form the product.





e.