Reactions of Alkenes Saturated hydrocarbons alkanes reacted by

Reactions of Alkenes

• Saturated hydrocarbons (alkanes) reacted by substitution, whereas the reaction of unsaturated hydrocarbons (alkenes) is by addition to the double bond

Addition Reactions • 1. Hydrogenation – addition of H 2 • - uses a heavy metal catalyst (Pt, Pd, or Ni) to add to the double bond • - the double bond breaks and the alkene is converted to an alkane • -both hydrogens are added from the same side (cis) of the double bond

Hydrogenation Examples • 1. Hydrogenate ethene. • 2. Hydrogenate 1, 2 -dimethylcyclopentene.

• 2. Halogenation - addition of Cl 2 or Br 2 • - this will always be a trans addition • Ex. ; 1. Halogenate (chlorinate) ethene. • 2. Brominate cyclopentene.

• NOTE: The addition of bromine is used as a qualitative test to see if a compound is saturated or unsaturated. • Bromine with CCl 4 reddish solution – Add this to a compound, if the red color persists, the compound is saturated (an alkane) – If the red color disappears, the compound is unsaturated (an alkene)

Using Unsymmetrical Reagents • Unsymmetrical reagents have two or more different atoms adding to the bond • Ex. ; H-OH, H-Br, H-Cl, H-HSO 4, CH 2=CH-CH 3, etc. • These follow Markownikoff’s Rule, which says when an unsymmetrical reagent adds to an unsymmetrical double bond, the positive part (usually the hydrogen) becomes attached to the double bonded carbon atom that has the greater number of hydrogens

• 3. Hydration – addition of water (H-OH) • -this requires the presence of an acid catalyst and yields an alcohol as the product • Ex. ; Hydrate propene.

• 4. Hydrohalogenation - addition of HI, HBr, HCl • Ex. ; 1. React propene with hydrochloric acid. • 2. React 2 -pentene with hydrobromic acid.

• 5. 1, 4 Addition – when two double bonds are separated by a single bond, they are said to be conjugated – Ex. ; CH 2=CH-CH=CH 2 • Addition will occur at the 1 and 4 positions relative to the double bond, and the double bond moves to the 2, 3 position • This works for any of the previous reagents • Ex. ; Brominate 1, 3 -butadiene

• 6. Oxidation - alkenes are readily oxidized, whereas alkanes are not • KMn. O 4 – potassium permanganate is the oxidizing agent; it changes from purple to brown indicating it was reduced and the alkene was oxidized • This is known as the Baeyer test – distinguishes alkanes (purple) from alkenes (brown) • The product depends on what type of alkene you start with • It is also a temperature dependent reaction

Oxidation Examples • 1. KMn. O 4 Δ • 2. • 3.

Oxidation Example – Temp. Dependent • 4. KMn. O 4 0°C

• 7. Ozonolysis - reaction with ozone (O 3) • Ozone is prepared by passing an electric charge through oxygen • Reacts rapidly with alkenes to give an aldehyde or a ketone • Breaks the alkene into two fragments at the double bond, each of which has an oxygen atom doubly bonded to the original alkene carbons • Used primarily to locate the position of a double bond

• Ex; 1. react 1 -butene with ozone. • 2. React 2 -butene with ozone. How is this reaction different than the first example