Chapter 7: Alkyl Halides and Nucleophilic Substitution Practice Cards

Comprehensive practice flashcards covering Chapter 7: Alkyl Halides and Nucleophilic Substitution, including nomenclature, stereochemistry, leaving groups, carbocations, and mechanisms (SN1 and SN2).

Primary ($1^\circ$) Alkyl Halide

An alkyl halide in which the halogen atom is bonded to a carbon atom that is directly attached to only one other carbon atom (e.g., structure I, 1-bromobutane).

Secondary ($2^\circ$) Alkyl Halide

An alkyl halide in which the halogen-bearing carbon is directly bonded to two other carbon atoms (such as 2-chloro-3,3-dimethylbutane).

Tertiary ($3^\circ$) Alkyl Halide

An alkyl halide in which the halogen-bearing carbon atom is directly bonded to three other carbon atoms (such as 1-fluoro-1-methylcyclohexane).

Vinyl Halide

A compound in which a halogen atom is bonded directly to a carbon-carbon double bond ($C=C$), an $sp^2$-hybridized carbon (such as vinyl bromide / bromoethene).

Allyl Halide vs. Benzyl Halide Common Names

Allyl bromide ($CH_2=CHCH_2Br$, structure II) contains a halogen bonded to a carbon adjacent to an alkene double bond; benzyl chloride ($C_6H_5CH_2Cl$, structure III) contains a chlorine bonded to a methyl carbon directly attached to a benzene ring.

Common Names: Vinyl Bromide and Isobutyl Chloride

Vinyl bromide ($CH_2=CHBr$, structure II) features bromine on an alkene carbon; isobutyl chloride ($(CH_3)_2CHCH_2Cl$, structure III) features a chlorine on a 2-methylpropyl group.

Common Names: Isobutyl Bromide and tert-Butyl Bromide

Isobutyl bromide ($(CH_3)_2CHCH_2Br$, structure III) is a primary halide with a branched isobutyl group; tert-butyl bromide ($(CH_3)_3CBr$, structure IV) is a tertiary halide with three methyl groups on the central carbon.

IUPAC Numbering Rule for Haloalkanes (e.g., 2-Chloro-4-methylpentane)

Number the longest carbon chain from the end closer to the first substituent; when groups are equidistant from opposite ends, assign the lower locant alphabetically (chloro at C2 before methyl at C4).

2-Bromo-5-methylheptane

The systematic IUPAC name for a seven-carbon alkane chain bearing a bromo substituent at position 2 and a methyl group at position 5.

2-Bromo-5,5-dimethylheptane

The IUPAC name for a heptane derivative where numbering from the end closer to the first substituent yields locants 2, 5, 5, alphabetized with bromo preceding dimethyl.

2-Chloro-4-isopropyl-2,6-dimethyloctane

The IUPAC systematic name for an octane chain substituted at C2 with chloro and methyl, at C4 with isopropyl, and at C6 with methyl.

(S)-3-Chloro-6-ethyloctane

The IUPAC stereochemical designation for an octane derivative where C3 bears a chlorine atom with an (S) absolute configuration and C6 bears an ethyl substituent.

trans-1-Chloro-4-methylcyclohexane

The IUPAC name for a disubstituted cyclohexane where the chloro substituent (wedge) and the methyl substituent (dash) reside on opposite faces of the ring.

(R)-3-Bromo-1-methylcyclohexene

The IUPAC stereochemical name for 3-bromo-1-methylcyclohexene, where the ring double bond receives locants 1 and 2, and the bromo stereocenter at C3 has an (R) configuration.

(Z)-2-Bromo-3,4-dimethyl-2-pentene

The IUPAC stereochemical name of an alkene where the higher-priority group on C2 ($-Br$) and the higher-priority group on C3 ($-\text{CH}(CH_3)_2$) reside on the same side of the double bond.

Dipole Moment Trend of Chloromethanes

The ranking of molecular polarity in chlorinated methanes: $CH_3Cl > CH_2Cl_2 > CHCl_3 > CCl_4$, where symmetrical carbon tetrachloride ($CCl_4$) is nonpolar ($\mu = 0$) because opposing bond dipoles cancel.

Polarity Trend of Halomethanes ($CH_3X$)

The order of increasing molecular polarity: $CH_4 < CH_3I < CH_3Br < CH_3Cl$ ($\text{I} < \text{IV} < \text{III} < \text{II}$), reflecting the increasing electronegativity of the halogen atom.

Characteristic Reactions of Alkyl Halides

The two primary and characteristic reaction pathways of alkyl halides: nucleophilic substitution and elimination.

Leaving Group Ability and Basicity

The conjugate bases of strong acids are weak bases and serve as good leaving groups in substitution and elimination reactions.

Leaving Group Ability Trend of Halides vs. Water

Decreasing order of leaving group ability: $I^- > Br^- > Cl^- > H_2O$ ($\text{III} > \text{II} > \text{I} > \text{IV}$), where iodide is the weakest base and the most effective leaving group among the halides.

Thermodynamic Favorability of Nucleophilic Substitution

Equilibrium favors substitution products when the departing leaving group is a weaker base than the attacking nucleophile.

Leaving Group Ability of Oxygen and Nitrogen Species

Increasing leaving group ability: $H_2N^- < HO^- < NH_3 < H_2O$ ($\text{IV} < \text{II} < \text{III} < \text{I}$), because weaker bases are superior leaving groups, and neutral leaving groups are weaker bases than their anionic counterparts.

Steric Hindrance in Bases vs. Nucleophiles

Steric crowding around a basic atom significantly decreases nucleophilicity because attack on an electrophilic carbon is hindered, but does not decrease basicity because proton transfer is unimpeded.

Nucleophilicity Trend in Polar Protic Solvents

Nucleophilicity increases down a column of the periodic table in polar protic solvents because larger anions are less tightly solvated by hydrogen bonding and can donate electrons more easily.

Polar Protic vs. Polar Aprotic Solvents

Polar protic solvents contain $O-H$ or $N-H$ bonds capable of hydrogen bonding (e.g., $H_2O$, alcohols), whereas polar aprotic solvents possess a dipole moment but lack hydrogen-bonding donor protons.

Halide Nucleophilicity in Polar Protic Solvents

Order of increasing nucleophilicity in protic media: $F^- < Cl^- < Br^- < I^-$ ($\text{I} < \text{II} < \text{III} < \text{IV}$), because the smaller fluoride anion is heavily caged by hydrogen-bonding solvent shells.

Most Nucleophilic Halide in Polar Aprotic Solvents

Fluoride ($F^-$), because polar aprotic solvents do not form hydrogen-bonding solvent cages around anions, allowing nucleophilicity to parallel basicity ($F^- > Cl^- > Br^- > I^-$).

Nucleophilicity Ranking: Oxygen vs. Sulfur Nucleophiles

Decreasing nucleophilicity order: $CH_3S^- > CH_3O^- > CH_3SH > CH_3OH$ ($\text{III} > \text{II} > \text{IV} > \text{I}$), because negatively charged species are more nucleophilic than neutrals, and sulfur is more polarizable than oxygen.

Periodic Trend in Neutral Nucleophiles (Second Row)

Ammonia ($NH_3$) is the most nucleophilic among neutral second-row hydrides ($CH_4$, $H_2O$, $NH_3$, $HF$) because nucleophilicity increases to the left across a period with decreasing electronegativity.

Nucleophilicity Ranking: Ethoxide and Ethanethiol Derivatives

Increasing nucleophilicity: $CH_3CH_2OH < CH_3CH_2SH < CH_3CH_2O^- < CH_3CH_2S^-$ ($\text{III} < \text{IV} < \text{I} < \text{II}$), since conjugate bases are stronger nucleophiles than parent molecules, and sulfur outcompetes oxygen.

Rate Effect of Nucleophile Concentration in an $S_N2$ Reaction

Because the $S_N2$ rate law is bimolecular ($\text{Rate} = k[\text{substrate}][\text{nucleophile}]$), doubling the nucleophile concentration doubles the overall reaction rate (factor of 2).

Effect of Doubling Both Reactants in an $S_N2$ Reaction

Doubling the concentrations of both the alkyl halide and the nucleophile in an $S_N2$ reaction increases the rate by a factor of 4 ($2 \times 2 = 4$).

Rate Effect of Nucleophile Concentration in an $S_N1$ Reaction

Because the rate-determining step in an $S_N1$ process is unimolecular ($\text{Rate} = k[\text{substrate}]$), doubling the concentration of the nucleophile leaves the rate unchanged.

Stereochemical Outcome of $S_N2$ Displacement

Bimolecular nucleophilic substitution proceeds via backside nucleophilic attack, resulting in complete inversion of configuration at an asymmetric stereocenter.

$S_N2$ Reaction Mechanism

A concerted, single-step reaction mechanism where nucleophilic bond formation and leaving group bond cleavage occur simultaneously with inversion of configuration.

Second-Order Substitution Susceptibility (Methyl Halides)

Bromomethane ($CH_3Br$) undergoes $S_N2$ substitution most readily because methyl halides lack alkyl steric hindrance, making backside attack fastest.

Leaving Group Influence on $S_N2$ Reaction Rate

1-Iodobutane undergoes the fastest $S_N2$ reaction because iodide ($I^-$) is the best leaving group among the halides due to its low basicity and weak $C-I$ bond.

$S_N2$ Reactivity Trend by Alkyl Substitution

Decreasing $S_N2$ reactivity order: 1-bromobutane ($1^\circ$, II) > 2-bromobutane ($2^\circ$, I) > 2-bromo-2-methylpropane ($3^\circ$, III), because steric hindrance increases with alkyl substitution, drastically impeding backside attack.

Solvent Effect on $S_N2$ Reaction Rate

Replacing a polar protic solvent ($CH_3OH$) with a polar aprotic solvent (DMSO, $(CH_3)_2S=O$) accelerates $S_N2$ reactions because nucleophiles are unsheltered by hydrogen bonding and become far more reactive.

Dimethylformamide (DMF)

A polar aprotic solvent ($HCON(CH_3)_2$, structure II) that readily solvates cations but leaves nucleophilic anions unencumbered and highly reactive.

Leaving Group Effect on $S_N1$ Rates

The rate of an $S_N1$ reaction increases as leaving group ability increases because cleavage of the $C-X$ bond occurs during the rate-determining step.

$S_N1$ Mechanism Characteristics

A two-step unimolecular substitution mechanism that proceeds through a carbocation intermediate and occurs fastest with tertiary ($3^\circ$) alkyl halides.

Substrate Reactivity in $S_N1$ Solvolysis

2-Chloro-2-methylbutane (structure IV) reacts fastest in $S_N1$ solvolysis because it is a tertiary alkyl halide, which ionizes to form a highly stabilized tertiary carbocation intermediate.

Rate-Determining Step in an $S_N1$ Mechanism

The heterolytic cleavage of the bond between the carbon atom and the leaving group to generate a carbocation intermediate.

$S_N1$ Kinetic Rate Law

$\text{Rate} = k[\text{alkyl halide}]$, a first-order rate law reflecting that only the alkyl substrate participates in the transition state of the rate-limiting step.

Tertiary Carbocation Intermediate Stability

Carbocation II (a tertiary 1-ethylcyclohexyl carbocation) is the most stable among the given structures because it has three alkyl groups donating electron density.

Carbocation Stability Ranking (Tertiary vs. Secondary)

Decreasing stability order: tertiary 1-methylcyclohexyl cation (I) > secondary 2-methylcyclohexyl cation (II) > secondary cyclohexyl cation (III), as greater alkyl substitution provides greater hyperconjugation and inductive stabilization.

Stereochemical Feature of $S_N1$ Reactions

$S_N1$ reactions proceed with racemization because the planar, achiral carbocation intermediate is attacked equally from either face, rather than producing complete inversion.

Electronic Factors Stabilizing Carbocations

Carbocations are stabilized by electron-donating inductive effects through adjacent $\sigma$ bonds and by hyperconjugation (overlap of adjacent filled $\sigma$ orbitals with the empty $p$ orbital).

Hammond Postulate Definition

A principle providing a qualitative estimate of transition state structure: the structure of a transition state resembles the species (reactants or products) to which it is closer in energy.

Hammond Postulate in Endothermic Processes

In an endothermic reaction (or endothermic step), the transition state is product-like in energy and structure, meaning that the pathway leading to the more stable product has lower activation energy ($E_a$) and forms faster.

$S_N2$ Transition State for Methoxide and 2-Bromopropane

A pentacoordinate transition state featuring partial bond formation to incoming methoxide ($CH_3O^{\delta-}$) and partial bond cleavage to departing bromide ($Br^{\delta-}$) on a secondary propyl carbon.

Solvolysis of 2-Chloro-2-methylbutane

Because 2-chloro-2-methylbutane is a tertiary alkyl halide reacting with a weak neutral nucleophile ($CH_3OH$), it undergoes an $S_N1$ mechanism, which proceeds faster in polar protic methanol ($CH_3OH$) than in DMSO.

Substitution of 1-Bromopropane with Hydroxide

Because 1-bromopropane is a primary alkyl halide reacting with a strong nucleophile ($HO^-$), substitution proceeds via an $S_N2$ mechanism, which is faster in polar aprotic DMF than in water.

Substitution of 3-Chloropentane with Methoxide

Because 3-chloropentane is a secondary alkyl halide reacting with a strong, unhindered nucleophile ($CH_3O^-$), the substitution pathway is $S_N2$, and it proceeds faster in the polar aprotic solvent HMPA than in methanol.

Inversion Product in $S_N2$ Substitution of (R)-2-Chlorobutane

Reaction of (R)-2-chlorobutane with methoxide in DMSO proceeds via an $S_N2$ mechanism with complete inversion of configuration to afford exclusively (S)-2-methoxybutane (structure I).

Inversion Product in $S_N2$ Substitution of (R)-2-Bromobutane

Reaction of (R)-2-bromobutane with sodium cyanide in acetone proceeds with inversion of stereochemistry to produce exclusively (S)-2-methylbutanenitrile (structure I).

Racemization in $S_N1$ Solvolysis of a Chiral Tertiary Halide

Solvolysis of a tertiary alkyl bromide in methanol generates an achiral planar carbocation that can be captured from either face, yielding a mixture of enantiomeric ether products (structures I and II).

Inversion in Substitution of (R)-2-Bromobutane with Methanethiol

Displacement of bromide by a sulfur nucleophile at a secondary stereocenter proceeds with Walden inversion to give the configurationally inverted thiol/thioether product (structure I).

Diastereomeric Products in $S_N1$ Solvolysis of 1-Bromo-1,3-dimethylcyclohexane

Solvolysis of chiral cis-1-bromo-1,3-dimethylcyclohexane in methanol generates a carbocation that is attacked from top and bottom faces, producing a mixture of diastereomers (structures I and II).

Stereochemical Inversion of trans-1-Bromo-3-methylcyclopentane

Reaction of trans-1-bromo-3-methylcyclopentane with cyanide ion ($CN^-$) proceeds by $S_N2$ inversion to yield cis-1-cyano-3-methylcyclopentane (structure II).

Precursor for trans-4-Methylcyclohexanol Synthesis

To synthesize trans-4-methylcyclohexanol via an $S_N2$ displacement with hydroxide ion, the starting material must have the opposite stereochemical configuration: cis-1-bromo-4-methylcyclohexane (structure III).

Precursor for 2-Cyclohexylacetonitrile Synthesis

(Bromomethyl)cyclohexane (structure IV), an unhindered primary alkyl halide that reacts efficiently with sodium cyanide via an $S_N2$ mechanism to add one carbon atom and form 2-cyclohexylacetonitrile.

Unreactivity of Aryl and Vinyl Halides in Nucleophilic Substitution

Aryl and vinyl halides do not undergo $S_N2$ reactions because the higher percent $s$-character of the $sp^2$ hybridized carbon makes the $C-X$ bond shorter and stronger, and backside attack is geometrically blocked; nor do they undergo $S_N1$ because $sp^2$ carbocations are extremely unstable.

Fastest Haloethane in $S_N2$ Substitution with Hydroxide

Iodoethane ($CH_3CH_2I$), because iodide is the largest halide, forms the weakest and most polarizable carbon-halogen bond, and acts as the best leaving group.

Optimal Solvent for $S_N2$ Alkylation with Methoxide

Dimethyl sulfoxide (DMSO), a polar aprotic solvent that solvates counterions without hydrogen bonding to the methoxide anion, maximizing its nucleophilicity.

Fastest Butyl Bromide Isomer in $S_N1$ Solvolysis

tert-Butyl bromide ($(CH_3)_3CBr$), because it ionizes to form a highly stabilized tertiary carbocation ($(CH_3)_3C^+$).

Ranking Haloalkanes by $S_N2$ Reactivity

Order of increasing $S_N2$ reactivity: 1-bromo-1-butene (vinyl, III) < 2-bromobutane ($2^\circ$, II) < 1-bromobutane ($1^\circ$, I) < 1-iodobutane ($1^\circ$ with superior leaving group, IV); that is, $\text{III} < \text{II} < \text{I} < \text{IV}$ (or decreasing: $\text{IV} > \text{I} > \text{II} > \text{III}$).

Ranking Haloalkanes by $S_N1$ Reactivity

Order of increasing $S_N1$ reactivity: 1-bromo-1-butene (vinyl, IV) < 1-bromobutane ($1^\circ$, I) < 2-bromobutane ($2^\circ$, II) < 2-bromo-2-methylbutane ($3^\circ$, III); that is, decreasing reactivity is $\text{III} > \text{II} > \text{I} > \text{IV}$, driven by the stability of the intermediate carbocation.