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Alkyl Halides and Elimination Reactions

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Updated 2010-05-14 18:45

General

StatementMissing Words
An elimination reaction introduces ___ into an organic compounda π bond
In an elimination reaction, an alkyl halide reacts with ___a Bronsted-Lowry base
In an elimination reaction, the elements of HX are lost and ___ is formedan alkene
Removal of the elements of HX is called ___dehydrohalogenation
Dehydrohalogenation is an example of ___β elimination
In β elimination, the base (B:) removes a proton on ___, thus forming H-B+the β carbon
In β elimination, ___ in the β C-H bond forms the new π bond between the α and β carbonsthe electron pair
In β elimination, the electron pair in ___ ends up on halogen, forming the leaving group :X-the C-X bond
In dehydrohalogenation, ___ is the sp3 hybridized carbon bonded to the leaving group Xthe α carbon
In a C=C double bond, ___, formed by end-on overlap of two sp2 hybrid orbitals, lies in the plane of the moleculea σ bond
In a C=C double bond, ___, formed by side-by-side overlap of two 2p orbitals, lies perpendicular to the plane of the moleculea π bond
___ has two groups on the same side of the C=C double bondthe cis isomer
___ has two groups on opposite sides of the C=C double bondthe trans isomer
In ___, a nucleophile attacks a single carbon atomnucleophilic substitution
In ___, a base attacks a protonβ elimination
In ___, a Bronsted-Lowry base removes a proton to form a π bond, and two carbons are involved in the reactionβ elimination
In both substitution and elimination, RX acts as ___, reacting with an electron-rich reagentan electrophile
Both substitution and elimination require ___ X:- that can accept the electron density in the C-X bonda good leaving group
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Common Bases Used in Dehydrohalogenation

FormulaName
Na+ -OHsodium hydroxide
K+ -OHpotassium hydroxide
Na+ -OCH3sodium methoxide
Na+ -OCH2CH3sodium ethoxide
K+ -OC(CH3)3potassium tert-butoxide
-OHhydroxide
-ORalkoxide
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Stability of Alkenes

StatementMissing Words
Cis alkenes are ___ stable than trans alkenesless
An alkene with more R groups bonded to the double bond carbons is ___ stablemore
Increasing the number of electron-donating R groups on a carbon atom able to accept electron density makes the alkene ___ stablemore
Trans alkenes have ___ steric interactions than cis alkenesless
Increasing alkyl substitution makes an alkene ___ stable by an electron-donating inductive effectmore
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E1 and E2 Elimination Mechanisms

StatementMissing Words
The ___ mechanism is unimolecular eliminationE1
The ___ mechanism is bimolecular eliminationE2
The ___ mechanism exhibits first-order kineticsE1
The ___ mechanism exhibits second-order kineticsE2
In the ___ mechanism, the concentration of the alkyl halide but not the base appears in the rate equationE1
In the ___ mechanism, the concentrations of both the alkyl halide and the base appear in the rate equationE2
The ___ mechanism is a two-step reaction: the bond to the leaving group breaks before the &pi bond is formedE1
The ___ mechanism is a concerted reaction: all bonds are broken and formed in a single stepE2
In an ___ reaction, the leaving group comes off before the β proton is removedE1
In an ___ reaction, the leaving group comes off while the β proton is removedE2
An ___ reaction is a one-step reactionE2
The most common mechanism for dehydrohalogenation is the ___ mechanismE2
DBN and DBU are two strong, sterically-hindered nitrogen bases useful for ___ reactionsE2
___ reactions are generally run with strong, negatively charged bases like -OH and -ORE2
___ reactions are favored by strong basesE2
___ reactions are favored by polar aprotic solventsE2
___ reactions are favored by weaker, neutral bases like H2O and ROHE1
___ reactions are favored by polar protic solventsE1
___ reactions need solvents that solvate the ionic intermediatesE1
___ reactions are regioselective and obey the Zaitsev rule, favoring formation of the more substituted, more stable alkeneboth E1 and E2
In an ___ reaction, a nucleophile attacks the carbocation, forming a subtitution productSN1
In an ___ reaction, a base removes a proton, forming a new π bondE1
___ reactions often occur with competing SN1 reactionsE1
For alkyl halides, of the E1 and E2 types, ___ reactions are much less usefulE1
Of the E1 and E2 types, ___ reactions are much more common and usefulE2
The ___ mechanism does not occur with 1o RX because they form highly unstable 1o carbocationsE1
The ___ mechanism has an anti periplanar arrangement of H and XE2
The ___ mechanism has a trigonal planar carbocation intermediateE1
In ___ reactions, the better the leaving group, the faster the reactionboth E1 and E2
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E2 Reactions

StatementMissing Words
The base -OH removes a proton from the ___, forming H2O (a by-product)β carbon
The electron pair in the β C-H bond forms the new ___π bond
The leaving group X- comes off with the ___ in the C-X bondelectron pair
The base appears in the rate equation, so the rate of the E2 reaction increases as the strength of the base ___increases
Because the bond to the leaving group is partially broken in the transition state, decreasing leaving group ability ___ the rate of the E2 reactiondecreases
The rate of an E2 reaction ___ in a polar aprotic solvent because such solvents do not solvate anions well and thus do not weaken negatively charged basesincreases
As the number of R groups on the carbon atom with the leaving group decreases, the rate of the E2 reaction ___decreases
Increasing the number of R groups on the carbon with the leaving group forms more highly substituted alkenes and ___ the stability of these alkenesincreases
From left-to-right, the rate of E2 reactions with R3CX, R2CHX, and RCH2X ___decreases
The Zaitsev rule states that the major product in β elimination has the more substituted ___double bond
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E1 Reactions

StatementMissing Words
It shares the same ___ step as the SN1 mechanismfirst
The ___ step is rate-determiningfirst
In the ___ step, heterolysis of the C-X bond forms an intermediate carbocationfirst
In the ___ step, a base (such as H2O or X-) removes a proton from a carbon adjacent to the carbocation (a β carbon)second
In the ___ step, the electron pair in the C-H bond is used to form the new π bondsecond
This mechanism obeys ___ order kineticsfirst
The ___ step is the fast stepsecond
SN1 and E1 reactions have exactly the same ___ step: formation of a carbocationfirst
The rate of an E1 reaction increases as the number of R groups on the carbon with the leaving group ___increases
From left-to-right, the rate of E1 reactions with R3CX, R2CHX, and RCH2X ___decreases
The rate of an E1 reaction increases as the carbocation stability ___increases
The reaction rate ___ for worse leaving groups because the bond to the leaving group is partially broken in the rate-determining stepdecreases
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E2 Reaction Stereochemistry

StatementMissing Words
The dihedral angle for the C-H and C-X bonds equals 0o for the ___ periplanar arrangementsyn
The dihedral angle for the C-H and C-X bonds equals 180o for the ___ periplanar arrangementanti
When the H and X atoms are oriented on the same side of the molecule, the geometry is called ___ periplanarsyn
When the H and X atoms are oriented on opposite sides of the molecule, the geometry is called ___ periplanaranti
E2 elimination occurs most often in the ___ periplanar geometryanti
___ periplanar geometry is the preferred arrangement for any alkyl halide undergoing E2 eliminationanti
The ___ periplanar arrangement has a staggered conformationanti
The ___ periplanar arrangement has an eclipsed conformationsyn
In the ___ periplanar arrangement, two electron-rich groups (the base and X) are far apartanti
In the ___ periplanar arrangement, two electron-rich groups (the base and X) are closesyn
In chlorocyclohexane, the C-Cl bond must be ___ periplanar to a C-H bond on a β carbon for E2 elimination to occuranti
In chlorocyclohexane with ___ Cl group, a β C-H bond and a C-Cl bond are never anti periplanar, so no E2 elimination occursan equatorial
In chlorocyclohexane with ___ Cl group, a β C-H bond and a C-Cl bond are trans diaxial, so E2 elimination occursan axial
In substituted cyclohexanes, E2 elimination must occur with ___ arrangement of H and X, and as a result, the more substituted alkene is not necessarily the major product (this is an exception to Zaitsev's rule)a trans diaxial
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E2 Reactions and Alkyne Synthesis

StatementMissing Words
The carbon-carbon ___ of an alkyne consists of one σ and two π bondstriple bond
The carbon-carbon ___ of an alkene consists of one σ bond and one π bonddouble bond
Alkynes are prepared by two successive ____ reactionsdehydrohalogenation
A ___ dihalide has two X atoms on adjacent carbon atomsvicinal
A ___ dihalide has two X atoms on the same carbon atomgeminal
The word ___ comes from the Latin word for twingeminal
sp hybridized carbons have ___ C-H bonds than sp2 hybridized carbonsstronger
sp3 hybridized carbons have ___ C-H bonds than sp2 hybridized carbonsweaker
___ bases are needed to synthesize alkynes by dehydrohalogenation than are needed to synthesize alkenesstronger
Because DMSO is a polar aprotic solvent, the anionic base is not well solvated, thus making the base ___stronger
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Is a reaction SN1, SN2, E1, or E2?

StatementMissing words
Good nucleophiles that are weak bases favor ___substitution
Bulky, nonnucleophilic bases favor ___elimination
Bases that are too sterically hindered to attack a tetravalent carbon, but are able to remove a small proton favor ___elimination
Reacting an alkyl halide with one of the anions I-, Br-, HS-, -CN, or CH3COO- favors ___substitution
Reacting an alkyl halide with one of KOC(CH3)3, DBU, or DBN favors ___elimination
3o alkyl halides (R3CX) react by all mechanisms except ___SN2
3o alkyl halides (R3CX) with strong bases undergo ___ reactionsE2
3o alkyl halides (R3CX) with weak nucleophiles and weak bases undergo ___ reactionsboth SN1 and E1
1o alkyl halides (RCH2X) undergo ___ reactionsboth SN2 and E2
1o alkyl halides (RCH2X) with strong nucleophiles undergo ___ reactionsSN2
1o alkyl halides (RCH2X) with strong, bulky, sterically hindered bases undergo ___ reactionsE2
2o alkyl halides (R2CHX) undergo ___ reactionsall 4 kinds of
2o alkyl halides (R2CHX) with strong bases and strong nucleophiles undergo ___ reactionsboth SN2 and E2
2o alkyl halides (R2CHX) with strong, bulky, sterically hindered bases undergo ___ reactionsE2
2o alkyl halides (R2CHX) with weak nucleophiles and weak bases undergo ___ reactionsboth SN1 and E1
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Reference: Chapter 8 of Janice Gorzynski Smith's "Organic Chemistry" 2nd Edition,
McGraw-Hill, 2008 (ISBN=978-0-07-304986-1).

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