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Alkyl Halides and Nucleophilic Substitution

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Updated 2010-05-14 18:57

General

StatementMissing Words
___ stands for a halogen atomX
Alkyl halides are often abbreviated as ___RX
Alkyl groups are often abbreviated as ___R
Nucleophiles are abbreviated _________:Nu-
Bronsted-Lowry bases are abbreviated ___:B
Alkyl halides have the general molecular formula ___CnH2n+1X
Vinyl halides have a halogen atom X bonded to ___a carbon-carbon double bond
Aryl halides have a halogen atom X bonded to ___a benzene ring
Allylic halides have a halogen atom X bonded to ___a carbon atom adjacent to a carbon-carbon double bond
Benzylic halides have a halogen atom X bonded to ___a carbon atom adjacent to a benzene ring
Vinyl and aryl halides have a halogen atom X directly bonded to ___an sp2 hybridized carbon atom
Allylic and benzylic halides have a halogen atom X directly bonded to ___an sp3 hybridized carbon atom
Alkyl halides are ___ moleculesweakly polar
Alkyl halides contain polar ___ bondsC-X
Alkyl halides are incapable of ___intermolecular hydrogen bonding
Alkyl halides have ___ boiling points and melting points than alkanes containing the same number of carbonshigher
Alkyl halides with larger surface area have ___ boiling points and melting pointshigher
Alkyl halides with less polar halogens have ___ boiling points and melting pointslower
Alkyl halides are soluble in ___organic solvents
Alkyl halides are insoluble in ___water
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StatementMissing Words
Alkyl halides undergo ___ reactions with nucleophilessubstitution
Alkyl halides undergo ___ reactions with Bronsted-Lowry baseselimination
R-X + :Nu- -> R-Nu + X:- is a general ___ reactionsubstitution
___ contain a lone pair or a π bond but not necessarily a negative chargenucleophiles
Nucleophilic substitutions are ___ reactionsLewis acid-base
___ attack protonsbases
___ attack other electron-deficient atoms (usually carbons)nucleophiles
A nucleophile is a ___ that reacts with electrophiles other than protonsLewis base
A ___ is a base that reacts with protonsBronsted-Lowry base
Nucleophiles and bases are structurally similar: both have a ___lone pair or a π bond
___ is the systematic preparation of a compound from a readily available starting material by one or many stepsorganic synthesis
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Leaving Groups

StatementMissing Words
In comparing two leaving groups, the better leaving group is the ___ baseweaker
Left-to-right across a row of the periodic table, basicity decreases so leaving group ability ___increases
Down a column of the periodic table, basicity ___ so leaving group ability increasesdecreases
From left-to-right the leaving group ability of F-, Cl-, Br-, and I- ___increases
From left-to-right, the basicity of NH3 and H2O ___decreases
Cl-, Br-, I-, and H2O are ___ leaving groupsgood
F-, -OH, -NH2, H-, and R- are ___ leaving groupspoor
Molecules with ___ leaving groups undergo nucleophilic substitutiongood
Molecules with ___ leaving groups do not undergo nucleophilic substitutionpoor
Equilibrium favors the products of nucleophilic substitution when the nucleophile is a ___ base than the leaving groupstronger
The ___ the conjugate acid, the stronger the baseweaker
The ___ the conjugate acid, the better the leaving groupstronger
The ___ the base, the better the leaving groupweaker
In general, ___ bases are stronger nucleophilesstronger
A negatively charged nucleophile is always ___ than its conjugate acidstronger
Left-to-right across a row of the periodic table, nucleophilicity ___ as basicity decreasesdecreases
Left-to-right, the basicity of F-, -OH, -NH2, and CH3- ___increases
Steric hindrance ___ nucleophilicity but not basicitydecreases
Sterically hindered bases that are ___ nucleophiles are called nonnucleophilic basespoor
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Solvent Effects

StatementMissing Words
___ solvents solvate both cations and anions wellpolar protic
___ solvents are capable of intermolecular hydrogen bondingpolar protic
___ are solvated by ion-dipole interactionscations
___ are solvated by hydrogen bondinganions
___ solvents are incapable of intermolecular hydrogen bondingpolar aprotic
___ solvents solvate only cations wellpolar aprotic
___ solvents contain O-H or N-H bondspolar protic
water, methanol, ethanol, tert-butanol, and acetic acid are ___ solventspolar protic
acetone, acetonitrile, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and hexamethylphosphoramine (HMPA) are ___ solventspolar aprotic
In ___ solvents, nucleophilicity rises with increasing size of an anion (opposite to basicity)polar protic
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SN1 and SN2 Reactions

StatementMissing Words
In the ___ mechanism, bond breaking and bond making occur simultaneouslyone-step
In the ___ mechanism, bond breaking occurs before bond makingtwo-step
An SN2 reaction follows the ___ mechanismone-step
An SN1 reaction follows the ___ mechanismtwo-step
___ stands for substitution nucleophilic bimolecularSN2
___ stands for substitution nucleophilic unimolecularSN1
An ___ reaction exhibits second-order kineticsSN2
An ___ reaction is bimolecularSN2
An ___ reaction's rate depends on the concentrations of both the alkyl halide and the nucleophileSN2
An ___ reaction exhibits first-order kineticsSN1
An ___ reaction is unimolecularSN1
An ___ reaction's rate-determining step depends only on the alkyl halide's concentrationSN1
All ___ reactions proceed with backside attack of the nucleophile, resulting in inversion of configuration at a stereogenic center SN2
Inversion of configuration in an ___ reaction is often called Walden inversionSN2
___ reactions proceed with racemization at a single stereogenic centerSN1
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SN2 Reactions

StatementMissing Words
As the number of R groups on the carbon with the leaving group increases, the rate of an SN2 reaction ___decreases
___ undergo SN2 reactions with easemethyl and 1o alkyl halides
___ react more slowly2o alkyl halides
___ do not undergo SN2 reactions3o alkyl halides
Increasing the number of R groups on the carbon with the leaving group ___ crowding in the transition state, decreasing the rate of an SN2 reactionincreases
The SN2 reaction is fastest with ___unhindered halides
Cells use ___ instead of alkyl halides such as CH3I in nucleophilic substitution reactionsSAM (s-adenosylmethionine)
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SN1 Reactions

StatementMissing Words
The SN1 mechanism has ___two steps
___ are formed as reactive intermediatescarbocations
The ___ is heterolysis of the C-X bondfirst step
The ___ is nucleophilic attack (a Lewis acid-base reaction)second step
The ___ is slowfirst step
The ___ is fastsecond step
The ___ is rate-determiningfirst step
The carbocation (with three groups around C) is ___ hybridized and trigonal planarsp2
The carbocation contains a vacant ___ orbital extending above and below the planep
___ from both sides of a planar carbocation occurs in SN1 reactionsnucleophilic attack
___ is the formation of equal amounts of two enantiomeric products from a single starting materialracemization
As the number of R groups on the carbon with the leaving group ___, the rate of an SN1 reaction increasesincreases
From left-to-right, the rate for SN1 reactions of R3CX, R2CHX, RCH2X, and CH3X ___decreases
___ undergo SN1 reactions rapidly3o alkyl halides
___ react more slowly2o alkyl halides
___ do not undergo SN1 reactionsmethyl and 1o alkyl halides
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Carbocation Stability

StatementMissing Words
As the number of ___ on the positively charged carbon atom increases, the stability of the carbocation increasesR groups
Inductive effects are electronic effects that occur through ___σ bonds
Electron donor groups (Z) stabilize ___cations
Electron-withdrawing groups (W) stabilize ___anions
Alkyl groups are ___ that stabilize a positive chargeelectron donor groups
Spreading out charge by the overlap of an empty p orbital with an adjacent σ bond is called ___hyperconjugation
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The Hammond Postulate

StatementMissing Words
The rate of an SN1 reaction increases as the number of R groups on the carbon with the leaving group ___increases
The stability of a carbocation decreases as the number of R groups on the positively charged carbon ___decreases
The rate of an SN1 reaction increases as the stability of the carbocation ___increases
In endothermic reactions, the transition state is closer in energy to the ___products
In exothermic reactions, the transition state is closer in energy to the ___reactants
Transition states in endothermic reactions resemble the ___products
Transition states in exothermic reactions resemble the ___reactants
In an ___ reaction, the more stable product forms fasterendothermic
In an ___ reaction, the more stable product may or may not form faster because Ea is similar for both productsexothermic
In the SN1 reaction, the rate-determining step is the formation of the carbocation, an ___ reactionendothermic
If the stability of the carbocation decreases, the rate of its formation ___decreases
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Predicting SN1 Versus SN2

StatementMissing Words
Increasing alkyl substitution favors ___SN1
Decreasing alkyl substitution favors ___SN2
Methyl and 1o halides (CH3X and RCH2X) undergo ___ reactions only SN2
3o alkyl halides (R3CX) undergo ___ reactions onlySN1
2o alkyl halides (R2CHX) undergo ___ reactionsboth SN1 and SN2
Strong nucleophiles present in high concentration favor ___ reactionsSN2
Weak nucleophiles favor ___ reactionsSN1
Weak nucleophiles decrease the rate of any competing ___ reactionSN2
RO- and -OH are ___ nucleophilesstrong
ROH and H2O are ___ nucleophilesweak
___ nucleophiles usually have a net negative chargestrong
___ nucleophiles are usually neutralweak
A better leaving group increases the rate of ___ reactionsboth SN1 and SN2
Polar protic solvents are especially good for ___ reactionsSN1
Polar aprotic solvents are especially good for ___ reactions SN2
Polar protic solvents favor ___ reactions because the ionic intermediates are stabilized by solvation SN1
Polar aprotic solvents favor ___ reactions because nucleophiles are not well solvated, and therefore are more nucleophilicSN2
The solvents H2O and CH3OH favor ___ reactionsSN1
The solvents acetone [(CH3)2C=O] and DMSO [(CH3)2S=O] favor ___ reactionsSN2
___ reactions occur only at sp3 hybridized carbon atomsboth SN1 and SN2
Vinyl halides and aryl halides both have a halogen atom bonded to an sp2 hybridized C and so do not undergo nucleophilic substitution by ___ reactionsboth SN1 and SN2
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Molecules Synthesized from R-X by SN2 Nucleophilic Substitution

Nucleophile (:Nu-)ProductName
-OHR-OHalcohol
-OR'R-OR'ether
-O-(C=O)-R'R-O-(C=O)-R'ester
-CNR-CNnitrile
-:C≡C-HR-C≡C-Halkyne
N3-R-N3azide
:NH3R-NH2amine
-SHR-SHthiol
-SR'R-SR'sulfide
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Reference: Chapter 7 of Janice Gorzynski Smith's "Organic Chemistry" 2nd Edition,
McGraw-Hill, 2008 (ISBN=978-0-07-304986-1).