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Reactions of alkenes. Lec.5. The E, Z System. A system to specify the configuration of groups about a carbon-carbon double bond. If three or four different groups attached to the carbon atoms of the double bond; E,Z system of nomenclature is applied.
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Reactions of alkenes Lec.5
The E, Z System • A system to specify the configuration of groups about a carbon-carbon double bond. • If three or four different groups attached to the carbon atoms of the double bond; E,Z system of nomenclature is applied. • FRIST, we decide which group on each carbon has the higher priority. • If the groups of higher priority are on the same side of the double bond, the configuration of the alkene is Z(German: zusammen, together). • If they are on opposite sides of the double bond, the alkene is E(German: entgegen, opposite).
Rules to determine the higher Priority: Br > Cl > O > N > C > H 35 17 8 7 6 1 The greater the atomic number, the higher is the priority. (Brhas a greater atomic number than H, so Br has a higher priority than H)
Reactions of alkenes There are three types of organic reactions: additions, eliminations and substitution. 1-Addition Reaction • The most characteristic reaction of alkenes is addition to the carbon-carbon double bond in such a way that the pibond is broken and, in its place, segmabonds (original double bond)to form two new atoms or groups of atoms • Reaction is nearly instantaneous, even at room temperature or below. No light or heat is required )
2-Elimination reactions: It is the opposite of addition reactions , that the single organic reactant splits into two products 3- Substitution reactions: It occurs when two reactants exchange parts to give two new products .
Addition reactions a) Addition of Halogens: Generally: Example:
b) Addition of Water (Hydration) If an acid catalyst is present, water adds to alkenes. It adds as H!OH, and the products are alcohols. • An acid catalyst is required in this case because the neutral water molecule is not acidic enough to provide protons to start the reaction • Hydration is used industrially and occasionally in the laboratory to synthesize alcohols from alkenes.
c )Addition of Acids The hydrogen ion (or proton) adds to one carbon of the double bond, and the remainder of the acid becomes connected to the other carbon.
Mechanism of Electrophilic Additionto Alkenes • What is an Electrophiles? • (literally, electron lovers) are electron-poor reagents; in reactions with some other molecule, they seek electrons. • They are often positive ions (cations). • NOTE: • Nucleophiles (literally, nucleus lovers), on the other hand, • are electron rich; they form bonds by donating electrons to an electrophile. “A reaction in which an electrophile is added to an alkene is called an electrophilic addition reaction”
The addition of acids to alkenes takes place on two steps: • 1- As the proton approaches the pi bond, the two pi electrons are used to form a sigma bond between the proton and one of the two carbon atoms. Because this bond uses both pi electrons, the other carbon acquires a positive charge, producing a carbon cation( carbocation).
2-The resulting carbocations are, however, extremely reactive because there are only six electrons (instead of the usual eight) around the positively charged carbon. The carbocation rapidly combines with some species that can supply it with two electrons(nucleophile.) The formation of the carbocation—is the slower of the two steps. The resulting carbocation is usually so reactive that combination with the nucleophile is extremely rapid. Since the first step in these additions is attack by the electrophile, the whole process is called an electrophilic addition reaction.
Reaction energy diagram for the addition of HBr to an alkene. • A reaction energy diagram shows the changes in energy that occur during the reaction. • A transition state is a structure with maximum energy for a particular • reaction step. • Activation energy, Ea, is the difference in energy between reactants and the transition state, and the activation • energy determines the reaction rate.
Markovnikov’s Rule Example, the addition of H-Clto propene. It may be :
The choice of addition location depends on the stability of carbocations So the addition of HX to any alkene, the hydrogen goes to the carbon with the most number of hydrogen atoms( i.e. less alkyl)
Addition of Hydrogen (hydrogenation) In the presence of a metal catalyst such as platinum, palladium, or nickel, hydrogen adds to the double bond of an alkene to form an alkane. The catalyst decreases the energy of activation by breaking the H- H bond. Examples:
