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Alkyl Halides and Elimination Reactions
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Updated
2010-05-14 18:45
General
Statement Missing Words An elimination reaction introduces ___ into an organic compound a 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 formed an 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 carbons the 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 X the α carbon In a C=C double bond, ___, formed by end-on overlap of two sp2 hybrid orbitals, lies in the plane of the molecule a 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 molecule a π bond ___ has two groups on the same side of the C=C double bond the cis isomer ___ has two groups on opposite sides of the C=C double bond the trans isomer In ___, a nucleophile attacks a single carbon atom nucleophilic substitution In ___, a base attacks a proton β elimination In ___, a Bronsted-Lowry base removes a proton to form a π bond, and two carbons are involved in the reaction β elimination In both substitution and elimination, RX acts as ___, reacting with an electron-rich reagent an electrophile Both substitution and elimination require ___ X:- that can accept the electron density in the C-X bond a good leaving group
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Common Bases Used in Dehydrohalogenation
Formula Name Na+ - OH sodium hydroxide K+ - OH potassium hydroxide Na+ - OCH3 sodium methoxide Na+ - OCH2 CH3 sodium ethoxide K+ - OC(CH3 )3 potassium tert-butoxide - OHhydroxide - ORalkoxide
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Stability of Alkenes
Statement Missing Words Cis alkenes are ___ stable than trans alkenes less An alkene with more R groups bonded to the double bond carbons is ___ stable more Increasing the number of electron-donating R groups on a carbon atom able to accept electron density makes the alkene ___ stable more Trans alkenes have ___ steric interactions than cis alkenes less Increasing alkyl substitution makes an alkene ___ stable by an electron-donating inductive effect more
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E1 and E2 Elimination Mechanisms
Statement Missing Words The ___ mechanism is unimolecular elimination E1 The ___ mechanism is bimolecular elimination E2 The ___ mechanism exhibits first-order kinetics E1 The ___ mechanism exhibits second-order kinetics E2 In the ___ mechanism, the concentration of the alkyl halide but not the base appears in the rate equation E1 In the ___ mechanism, the concentrations of both the alkyl halide and the base appear in the rate equation E2 The ___ mechanism is a two-step reaction: the bond to the leaving group breaks before the &pi bond is formed E1 The ___ mechanism is a concerted reaction: all bonds are broken and formed in a single step E2 In an ___ reaction, the leaving group comes off before the β proton is removed E1 In an ___ reaction, the leaving group comes off while the β proton is removed E2 An ___ reaction is a one-step reaction E2 The most common mechanism for dehydrohalogenation is the ___ mechanism E2 DBN and DBU are two strong, sterically-hindered nitrogen bases useful for ___ reactions E2 ___ reactions are generally run with strong, negatively charged bases like - OH and - OR E2 ___ reactions are favored by strong bases E2 ___ reactions are favored by polar aprotic solvents E2 ___ reactions are favored by weaker, neutral bases like H2 O and ROH E1 ___ reactions are favored by polar protic solvents E1 ___ reactions need solvents that solvate the ionic intermediates E1 ___ reactions are regioselective and obey the Zaitsev rule, favoring formation of the more substituted, more stable alkene both E1 and E2 In an ___ reaction, a nucleophile attacks the carbocation, forming a subtitution product SN 1 In an ___ reaction, a base removes a proton, forming a new π bond E1 ___ reactions often occur with competing SN 1 reactions E1 For alkyl halides, of the E1 and E2 types, ___ reactions are much less useful E1 Of the E1 and E2 types, ___ reactions are much more common and useful E2 The ___ mechanism does not occur with 1o RX because they form highly unstable 1o carbocations E1 The ___ mechanism has an anti periplanar arrangement of H and X E2 The ___ mechanism has a trigonal planar carbocation intermediate E1 In ___ reactions, the better the leaving group, the faster the reaction both E1 and E2
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E2 Reactions
Statement Missing Words The base - OH removes a proton from the ___, forming H2 O (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 bond electron 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 reaction decreases 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 bases increases 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 alkenes increases From left-to-right, the rate of E2 reactions with R3 CX, R2 CHX, and RCH2 X ___ decreases The Zaitsev rule states that the major product in β elimination has the more substituted ___ double bond
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E1 Reactions
Statement Missing Words It shares the same ___ step as the SN 1 mechanism first The ___ step is rate-determining first In the ___ step, heterolysis of the C-X bond forms an intermediate carbocation first In the ___ step, a base (such as H2 O 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 bond second This mechanism obeys ___ order kinetics first The ___ step is the fast step second SN 1 and E1 reactions have exactly the same ___ step: formation of a carbocation first 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 R3 CX, R2 CHX, and RCH2 X ___ 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 step decreases
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E2 Reaction Stereochemistry
Statement Missing Words The dihedral angle for the C-H and C-X bonds equals 0o for the ___ periplanar arrangement syn The dihedral angle for the C-H and C-X bonds equals 180o for the ___ periplanar arrangement anti When the H and X atoms are oriented on the same side of the molecule, the geometry is called ___ periplanar syn When the H and X atoms are oriented on opposite sides of the molecule, the geometry is called ___ periplanar anti E2 elimination occurs most often in the ___ periplanar geometry anti ___ periplanar geometry is the preferred arrangement for any alkyl halide undergoing E2 elimination anti The ___ periplanar arrangement has a staggered conformation anti The ___ periplanar arrangement has an eclipsed conformation syn In the ___ periplanar arrangement, two electron-rich groups (the base and X) are far apart anti In the ___ periplanar arrangement, two electron-rich groups (the base and X) are close syn In chlorocyclohexane, the C-Cl bond must be ___ periplanar to a C-H bond on a β carbon for E2 elimination to occur anti In chlorocyclohexane with ___ Cl group, a β C-H bond and a C-Cl bond are never anti periplanar, so no E2 elimination occurs an equatorial In chlorocyclohexane with ___ Cl group, a β C-H bond and a C-Cl bond are trans diaxial, so E2 elimination occurs an 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
Statement Missing Words The carbon-carbon ___ of an alkyne consists of one σ and two π bonds triple bond The carbon-carbon ___ of an alkene consists of one σ bond and one π bond double bond Alkynes are prepared by two successive ____ reactions dehydrohalogenation A ___ dihalide has two X atoms on adjacent carbon atoms vicinal A ___ dihalide has two X atoms on the same carbon atom geminal The word ___ comes from the Latin word for twin geminal sp hybridized carbons have ___ C-H bonds than sp2 hybridized carbons stronger sp3 hybridized carbons have ___ C-H bonds than sp2 hybridized carbons weaker ___ bases are needed to synthesize alkynes by dehydrohalogenation than are needed to synthesize alkenes stronger 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 SN 1, SN 2, E1, or E2?
Statement Missing 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 CH3 COO- favors ___ substitution Reacting an alkyl halide with one of KOC(CH3 )3 , DBU, or DBN favors ___ elimination 3o alkyl halides (R3 CX) react by all mechanisms except ___ SN 2 3o alkyl halides (R3 CX) with strong bases undergo ___ reactions E2 3o alkyl halides (R3 CX) with weak nucleophiles and weak bases undergo ___ reactions both SN 1 and E1 1o alkyl halides (RCH2 X) undergo ___ reactions both SN 2 and E2 1o alkyl halides (RCH2 X) with strong nucleophiles undergo ___ reactions SN 2 1o alkyl halides (RCH2 X) with strong, bulky, sterically hindered bases undergo ___ reactions E2 2o alkyl halides (R2 CHX) undergo ___ reactions all 4 kinds of 2o alkyl halides (R2 CHX) with strong bases and strong nucleophiles undergo ___ reactions both SN 2 and E2 2o alkyl halides (R2 CHX) with strong, bulky, sterically hindered bases undergo ___ reactions E2 2o alkyl halides (R2 CHX) with weak nucleophiles and weak bases undergo ___ reactions both SN 1 and E1
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Reference: Chapter 8 of Janice Gorzynski Smith's "Organic Chemistry" 2nd Edition,
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