Chemca Formula Sheet - Haloalkanes

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EXAM MASTER FORMULA SHEET

Organic Chemistry: Haloalkanes

Substitutions, Eliminations & Grignard Reagents

1. Nature of C-X Bond & Properties

Haloalkanes ($\ce{R-X}$) contain a highly polarized covalent bond due to the electronegativity difference between Carbon and Halogen.

Bond Strength & Length

Bond Length: $\ce{C-I > C-Br > C-Cl > C-F}$

Bond Enthalpy: $\ce{C-F > C-Cl > C-Br > C-I}$

Reactivity $\propto$ 1 / Bond Strength
($\ce{R-I}$ is most reactive)

Dipole Moment & Boiling Point

Dipole Anomaly: $\ce{CH3Cl > CH3F > CH3Br > CH3I}$
($\ce{CH3Cl}$ has higher dipole than $\ce{CH3F}$ due to longer bond length).

B.P. Order: $\ce{R-I > R-Br > R-Cl > R-F}$
(B.P. $\propto$ Mol. Mass & Surface Area). Straight chain > Branched.

2. Methods of Preparation

From Alcohols (Nucleophilic Substitution)

Lucas Reagent (For Chlorides):

$\ce{R-OH + HCl ->[ZnCl2] R-Cl + H2O}$

$3^\circ$ alcohols react instantly (turbidity).

Darzen's Process (Best Method):

$\ce{R-OH + SOCl2 -> R-Cl + SO2 \uparrow + HCl \uparrow}$

Byproducts are escaping gases. Retains configuration ($S_Ni$) unless Pyridine is used ($S_N2$ inversion).

Halogen Exchange Methods

Finkelstein Reaction (For Iodides):

$\ce{R-X + NaI ->[Dry Acetone] R-I + NaX v}$

$\ce{NaX}$ ($\ce{X=Cl, Br}$) precipitates in acetone, driving reaction forward (Le Chatelier).

Swarts Reaction (For Fluorides):

$\ce{R-X + AgF -> R-F + AgX v}$

Other heavy metal fluorides ($\ce{Hg2F2, CoF3, SbF3}$) can also be used.

Addition to Alkenes:

Markovnikov Addition: $\ce{R-CH=CH2 + HX \to R-CH(X)-CH3}$ (Via Carbocation).
Anti-Markovnikov (Kharasch Effect): Valid ONLY for $\ce{HBr}$ in presence of peroxides. Proceeds via Free Radical mechanism to give primary bromide.

3. Nucleophilic Substitution ($S_N1$ vs $S_N2$)

Feature	S$_N$1 (Unimolecular)	S$_N$2 (Bimolecular)
Kinetics & Rate	Rate $= k[\ce{R-X}]$	Rate $= k[\ce{R-X}][Nu^-]$
Mechanism	2-step. Intermediate is a Carbocation. (Rearrangements possible!).	1-step concerted. Intermediate is a Pentacoordinate Transition State.
Stereochemistry	Racemization (Net slight inversion due to ion-pair)	100% Walden Inversion (Backside attack by nucleophile)
Substrate Reactivity	Benzyl/Allyl > $3^\circ > 2^\circ > 1^\circ > \ce{CH3}$	$\ce{CH3} > 1^\circ > 2^\circ > 3^\circ$ (Steric hindrance dominates)
Favored Solvent	Polar Protic ($\ce{H2O, EtOH, HCOOH}$) Stabilizes both carbocation and leaving group.	Polar Aprotic (Acetone, DMF, DMSO) Solvates cation only, leaving nucleophile naked.

Ambidentate Nucleophiles: Reagents like $\ce{KCN}$ yield Cyanides ($\ce{R-CN}$) because the $\ce{C-C}$ bond is stronger than $\ce{C-N}$. However, $\ce{AgCN}$ is covalent, so attack occurs through Nitrogen to yield Isocyanides ($\ce{R-NC}$). Similarly for $\ce{KNO2}$ (Nitrite) vs $\ce{AgNO2}$ (Nitro).

4. Elimination & Organometallics

Dehydrohalogenation ($\beta$-Elimination)

When an alkyl halide is heated with Alcoholic KOH, a molecule of HX is eliminated to form an alkene. It follows the E2 mechanism.

Saytzeff (Zaitsev) Rule: Major product is the highly substituted (most stable) alkene. Standard for most small bases ($EtO^-$).

Hofmann Rule: Major product is the least substituted alkene. Occurs with bulky bases ($t-BuO^-$) or poor leaving groups ($\ce{F^-}$).

Grignard Reagent ($\ce{R-Mg-X}$)

$\ce{R-X + Mg ->[Dry Ether] R-Mg-X}$

The $\ce{C-Mg}$ bond is highly polar. Grignard reagents are powerful nucleophiles and very strong bases.

Moisture Sensitivity:

$\ce{RMgX + H2O -> R-H + Mg(OH)X}$

Wurtz Reaction Limitations

Couples two alkyl radicals to form a higher, symmetrical alkane. Unsuitable for unsymmetrical alkanes (gives mixture).

$\ce{2RX + 2Na ->[Dry Ether] R-R + 2NaX}$

Tertiary halides undergo elimination (alkene) rather than coupling.

5. Essential Stereochemistry Terms

Chirality

A molecule that is non-superimposable on its mirror image. Usually requires an asymmetric carbon ($sp^3$ attached to 4 different groups).

Enantiomers

Stereoisomers that are perfect, non-superimposable mirror images. They have identical physical properties but rotate plane-polarized light in opposite directions.

Diastereomers

Stereoisomers that are not mirror images of each other. They have different physical properties (B.P., M.P., solubility) and can be separated easily.

Meso / Racemic

Meso: Optically inactive due to internal symmetry.
Racemic Mixture: 50:50 mixture of enantiomers; optically inactive due to external compensation.

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Exam Master Review Sheet - Haloalkanes