Organic 5: Structure & Preparation of Alkenes - Elimination Reactions

Question 1

Alkenes

Answer 1

hydrocarbons that contain a carbon-carbon double bond

Question 2

Alkenyl groups

Answer 2

vinyl, allyl, isopropenyl

Question 3

Cycloalkenes

Answer 3

these and their derivatives are named by adapting cycloalkane terminology to the principles of alkene nomenclature

Question 4

Dehydrogenation

Answer 4

elimination in which H2 is lost from adjacent atoms; the term is most commonly encountered in the industrial preparation of ethylene from ethane, propene from propane, 1,3-butadiene from butane, and styrene from ethylbenzene

Question 5

Zusammen

Answer 5

German meaning “together”; denotes when atoms of higher atomic number are on the same side of the double bond (Z configuration)

Question 6

Entgegen

Answer 6

German meaning “opposite”; denotes when atoms of higher atomic number are on opposite sides of the double bond (E configuration)

Question 7

Monosubstituted

Answer 7

an alkene of the type RCH=CH2, in which there is only 1 carbon directly bonded to the carbons of the double bond

Question 8

Disubstituted

Answer 8

alkene of the type R2C=CH2 or RCH=CHR; the groups R may be the same or different, they may be any length, and they may be branched or unbranched; the significant point is that there are 2 carbons directly bonded to the carbons of the double bond

Question 9

trisubstituted

Answer 9

alkene of the type R2C=CHR, in which there are 3 carbons directly bonded to the carbons of the double bond (the R groups may be the same or different

Question 10

tetrasubstituted

Answer 10

alkene of the type R2C=CR2, in which there are 4 carbons directly bonded to the carbons of the double bond (the R groups may be the same or different)

Question 11

Electronic effect

Answer 11

an effect that results when 2 or more atoms or groups interact so as to alter the electron distribution in a system; the greater stability of more highly substituted alkenes is an example of this

Question 12

Steric effect

Answer 12

an effect that results when 2 or more atoms are close enough in space that a repulsion occurs between them is 1 type of this

Question 13

Beta elimination reactions (1,2 eliminations)

Answer 13

reaction in which a double or triple bond is formed by loss of atoms or groups from adjacent atoms

Question 14

Dehydration

Answer 14

removal of H and OH from adjacent atoms; the term is most commonly employed in the preparation of alkenes by heating alcohols in the presence of an acid catalyst

Question 15

Regioselective

Answer 15

reactions that can proceed in more than one directions, but in which one direction is preferred

Question 16

Zaitsev’s rule

Answer 16

the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the beta carbon having the fewest hydrogens; often expressed as “beta elimination reactions of alcohols yield the most highly substituted alkene as the major product”; preference for predominant formation of the most stable alkene that could arise by beta elimination

Question 17

Stereoselective

Answer 17

reaction in which a single starting material can yield 2 or more stereoisomeric products, but gives one of them in greater amounts than any other

Question 18

E1 (Elimination unimolecular mechanism)

Answer 18

mechanism for elimination characterized by the slow formation of a carbocation intermediate followed by rapid loss of a proton from the carbocation to form the alkene

Question 19

E2 (Elimination bimolecular mechanism)

Answer 19

mechanism for elimination of alkyl halides characterized by a transition state in which the attacking base removes a proton at the same time that the bond to the halide leaving group is broken

Question 20

Rearrangement

Answer 20

intramolecular migration of an atom, a group, or a bond from one atom to another

Question 21

Hydride shifts

Answer 21

migration of a hydrogen with a pair of electrons (H:) from one atom to another; most commonly seen in carbocation rearrangements

Question 22

Dehydrohalogenation

Answer 22

loss of a hydrogen and a halogen from an alkyl halide; one of the most useful methods for preparing alkenes by beta elimination

Question 23

Leaving group

Answer 23

the group, normally a halide ion, that is lost from carbon in a nucleophilic substitution or elimination

Question 24

Stereoelectronic effect

Answer 24

effects that arise because one spatial arrangement of electrons (or orbitals or bonds) is more stable than another

Question 25

Kinetic isotope effect

Answer 25

an effect on reaction rate that depends on isotopic composition

Question 26

Deuterium isotope effect

Answer 26

the difference in a property, usually reaction rate, that results when one or more atoms of 1H in a compound are replaced by 2H