Addition of HX to an Unsymmetrical Alkene Why????

Reaction Mechanism 1. A proton (H+) from HCl bonds to carbon 1 of propene by utilizing the pi bond electrons. The intermediate formed is a positively charged alkyl group, or carbocation. The positive charge is localized on carbon 2 of this carbocation.

Reaction Mechanism 2. The chloride ion then adds to the positively charged carbon atom to form a molecule of 2-chloropropane.

Carbocation An ion in which a carbon atom has a positive charge is known as a carbocation.

The order of stability of carbocations and hence the ease with which they are formed is:

Why??? Markovnikov’s Rule When an unsymmetrical molecule such as HX (HCl) adds to a carbon-carbon double bond, the hydrogen from HX goes to the carbon atom that has the greater number of hydrogen atoms. Why???

Markovnikov’s Rule This reaction proceeds via the formation of the most stable carbocation intermediate (2°).

Write formulas for the organic products formed when 2-methyl-1-butene reacts with: H2, Pt/25°C Cl2 HCl H20, H+

2-methyl-1-butene + H2, Pt/25 °C

2-methyl-1-butene + Cl2

2-methyl-1-butene + HCl

2-methyl-1-butene + H2O

Oxidation

Oxidation at the C=C Bond Baeyer Test

Alkynes: Nomenclature and Preparation The rules for naming alkynes are the same as those for alkenes, but the ending –yne is used to indicate the presence of a triple bond.

Preparation of Alkynes Acetylene can be prepared from calcium carbide and water. CaC2 + 2H2O  HCCH + Ca(OH)2 Acetylene is also prepared by the cracking of methane at 1500 °C. 2CH4  HCCH + 3H2

Physical and Chemical Properties of Alkynes

Physical Properties of Alkynes Acetylene is a colorless gas with little odor when pure. Acetylene is insoluble in water and is a gas at normal temperature and pressure.

Chemical Properties of Alkynes Alkynes undergo addition reactions rather similar to those of alkenes. Cl2 and Br2 HCl and HBr Positive reaction with Baeyer’s test.

Bromination of Acetylene HCCH + Br2  CHBr=CHBr HCCH + 2 Br2  CHBr2-CHBr2

HCl Addition to Unsymmetrical Alkynes This addition follows Markovnikov’s rule: CH3CCH + HCl  CH3CCl=CH2 CH3CCH + 2 HCl  CH3CCl2-CH3

Aromatic Hydrocarbons: Structure

Benzene Benzene and all substances with structures and chemical properties that resemble benzene are classified as aromatic compounds.

Bonding in Benzene The electrons are not attached to particular carbon atoms, but are delocalized and associated with the entire molecule. This electronic structure imparts unusual stability to benzene and is responsible for many of the characteristic properties of aromatic compounds.

Bonding in Benzene Figure 20.5 (a) sp2-sp2 orbital overlap to form the carbon ring structure.

Bonding in Benzene Figure 20.5 (b) carbon-hydrogen bonds formed by sp2-s orbital overlap and overlapping p orbitals.

Bonding in Benzene Figure 20.5 (c) pi electron clouds above and below the plane of the carbon ring.

Naming Aromatic Compounds

Naming Substituted Benzene Compounds A substituted benzene is derived by replacing one or more hydrogen atoms of benzene by another atom or group of atoms. Monosubstituted benzene has the formula C6H5G, where G is the group replacing a hydrogen atom.

Monosubstituted Benzenes Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene.

Certain monosubstituted benzenes have special names.

Phenyl Group The C6H5- group is known as the phenyl group, and the name phenyl is used to name compounds that cannot easily be named as benzene derivatives.

Disubstituted Benzenes The prefixes ortho-, meta-, and para- (abbreviated o-, m-, and p-) are used to name disubstituted benzenes.

Dichlorobenzenes, C6H4Cl2 The three isomers of dichlorobenzene have different physical properties.

Disubstituted Benzenes When the two substituents are different and neither is part of a compound with a special name, the names of the two substituents are given in alphabetical order, followed by the word benzene.

Dimethyl Benzenes The dimethylbenzenes have the special name xylene.

Disubstituted Benzenes When one of the substituents corresponds to a monosubstituted benzene that has a special name, the disubstituted compound is named as a derivative of that parent compound.

Polysubstituted Benzenes When there are more than two substituents on a benzene ring, the carbon atoms in the ring are numbered starting at one of the substituted groups. Numbering must be done in the direction that gives the lowest possible numbers to the substituent groups.

Polysubstituted Benzenes

Polycyclic Aromatic Compounds

Polycyclic Aromatic Hydrocarbons

Sources and Physical Properties of Aromatic Hydrocarbons

Sources of Aromatic Hydrocarbons The aromatic hydrocarbons, such as benzene, toluene, xylene, naphthalene, and anthracene, were first obtained in significant quantities from coal tar. Coal  Coke + Coal gas + Coal tar Because of the great demand for aromatic hydrocarbons, processes were devised to obtain them from petroleum. 

Properties of Aromatic Hydrocarbons Aromatic hydrocarbons are essentially nonpolar substances, insoluble in water but soluble in many organic solvents. They are liquids or solids and usually have densities less than that of water. Aromatic hydrocarbons burn readily, usually with smoky yellow flames as a result of incomplete carbon combustion.

Chemical Properties of Aromatic Hydrocarbons

Substitution Reactions of Aromatic Hydrocarbons Halogenation net addition of -Br or -Cl Nitration net addition of –NO2 Alkylation net addition of –R (alkyl group)

Halogenation of Benzene When benzene reacts with chlorine or bromine in the presence of a catalyst such as iron (III) chloride or iron (III) bromide, a Cl or Br atom replaces an H atom to form the products.

Nitration of Benzene When benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid at about 50C, nitrobenzene is formed.

Alkylation of Benzene Alkylation of benzene is known as the Friedel-Crafts reaction. The alkyl group from an alkyl halide (RX), in the presence of AlCl3 catalyst, substitutes for an H atom on the benzene ring.

Side-Chain Oxidation Carbon chains attached to an aromatic ring are fairly easy to oxidize.