Some more Reactions of Alkanes

A. Oxidation The oxidation of alkanes by O2 to give carbon dioxide and water is by far their most economically important reaction. Oxidation of saturated hydrocarbons is the basis for their use as energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power (gasoline, diesel fuel, and aviation fuel). Following are balanced equations for the complete oxidation of methane (the major component of natural gas) and propane (the major component of LPG). CH4 1 2O2 9: CO2 12H2O ∆H° 5 2890.4 kJ (2212.8 kcal)/mol Methane CH3CH2CH3 1 5O2 3CO2 14H2O ∆ H° 5 22220 kJ (2530.6 kcal)/mol

The most characteristic reaction of alkenes is addition to the carbon-carbon double bond in such a way that the π bond is broken and, in its place, σ bonds form to two new atoms or groups of atoms Hydrochlorination (hydrohalogenation) Hydration Bromination (halogenation) Hydrogenation (reduction)

Some more addition reactions Oxymercuration Mercury (II) acetate Diethyl ether Product iis alcohol

Diol formation (oxidation)

Electrophilic Additions The overall reaction of electrophilic species adding to π bonds is called electrophilic addition The key step in these mechanisms involves the π electrons of the alkene reacting as a nucleophile with an electrophilic species in a bond-making step. This mechanistic element as make a bond between a nucleophile (p bond) and an electrophile

Kinds of electrophiles Full positive charge There are at least three characteristic features that can make a molecule or ion electrophilic. The most common of these features is a region of low electron density, reflected by a partial or full positive charge. The full or partial positive charge aids reaction with an electron rich region of a nucleophile through electrostatic attraction.

A partial positive charge with an acidic hydrogen

Boron atom lacking an octet

A positive charge on a carbon atom that is lacking an octet

A third characteristic that can make a molecule or ion electrophilic is a relatively weak bond to an atom that can depart as a stable ion or molecule

Regioselective reaction A regioselective reaction is a reaction in which one direction of bond forming or breaking occurs in preference to all other directions of bond forming or breaking. Unsymmetrical Alkene This regioselectivity was noted by Vladimir Markovnikov who made the generalization known as Markovnikov’s rule: in the addition of H9X to an alkene, hydrogen adds to the double-bonded carbon that has the greater number of hydrogens already bonded to it

Examples Name and draw a structural formula for the product of each alkene addition reaction:

Mechanism Electrophilic Addition of HBr to 2-Butene

Mechanism Electrophilic Addition of HBr to 2-Butene

Step 1: Make a new bond between a nucleophile (π-bond) and an electrophile— add a proton. Addition begins with the transfer of a proton from H-Br to 2-butene, as shown by the two curved arrows on the left side of the equation. The first curved arrow shows that the π-bond of the alkene breaks and its electron pair forms a new covalent bond with the hydrogen atom of H-Br. The second curved arrow shows that the polar covalent bond in H-Br breaks and its electron pair moves to bromine to form a bromide ion. The result of this step is the formation of an organic cation and bromide ion.

Step 2: Make a new bond between a nucleophile and an electrophile. Reaction of the sec-butyl cation (an electrophile) with bromide ion (a nucleophile) completes the valence shell of carbon and gives 2-bromobutane

Two-step mechanism Step 1, the addition of a proton to the alkene, results in formation of a cationic intermediate A species containing a positively charged carbon atom is called a carbocation (carbon 1 cation). First step is slow step and it is called Rate determining step. Second step is fast step

Energy profile