Organic Chemistry: Alkyl Halides

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Questions and Answers

What type of alkyl halide contains two halogen atoms bonded to adjacent carbon atoms?

Vicinal dihalide (correct)

Geminal dihalide

TriHaloalkane

MonoHaloalkane

Which of the following describes a primary alkyl halide?

The halogen is bonded to a carbon that is connected to another carbon with two halogen atoms.

The halogen is bonded to a carbon connected to three other carbons.

The halogen is bonded to a carbon connected to only one other carbon. (correct)

The halogen is bonded to a carbon in a cyclic structure.

Which prefix is used to denote an alkyl halide with a fluorine substituent?

Fluoro- (correct)

Bromo-

Iodo-

Chloro-

What alkyl halide type is characterized by one halogen atom?

MonoHaloalkane Signup and view all the answers

What is the common use of carbon tetrachloride (CCl4)?

In dry cleaning and spot removing. Signup and view all the answers

Which of the following compounds is identified as a haloform?

CHI3 Signup and view all the answers

What distinguishes a benzylic halide from other alkyl halides?

The halogen is bonded to a benzylic carbon connected to an aromatic ring. Signup and view all the answers

Which of the following is a characteristic of tertiary alkyl halides?

They have the halogen atom connected to a carbon with at least two other carbon atoms. Signup and view all the answers

Which type of carbon hybridization is present in alkyl halides?

sp3 Signup and view all the answers

What is the primary reason for the insolubility of alkyl halides in water?

Inability to form hydrogen bonds Signup and view all the answers

Which of the following alkyl halides would have the highest boiling point?

CH3CH2I Signup and view all the answers

Which statement correctly reflects the properties of alkyl halides?

Higher molecular weights generally lead to higher boiling points in alkyl halides. Signup and view all the answers

What role do London forces play in the boiling points of alkyl halides?

They are the strongest intermolecular attraction in alkyl halides. Signup and view all the answers

How does the presence of a larger carbon chain affect the melting point of alkyl halides?

It increases the melting point. Signup and view all the answers

Which alkyl halide has the following nomenclature: CH3CH2Br?

Bromoethane Signup and view all the answers

What can be inferred about the density of alkyl halides compared to water?

Alkyl iodides and bromides are less dense than water. Signup and view all the answers

Which statement accurately describes the classification of alkyl halides?

Alkyl halides are exclusively classified as primary, secondary, or tertiary based on the halogen replacement. Signup and view all the answers

What characteristic property is often observed in halogenated organic compounds?

Many are non-polar and have lower solubility in polar solvents. Signup and view all the answers

How does the addition of halogen acids to alkenes follow Markovnikov's rule?

The halogen atom attaches to the carbon with fewer hydrogen atoms. Signup and view all the answers

What is the key reason for using N-bromosuccinamide (NBS) in allylic bromination reactions?

NBS continually generates Br2, facilitating the reaction. Signup and view all the answers

In which reaction do alcohols produce alkyl bromides or alkyl iodides?

Action of halogen acids. Signup and view all the answers

Which of these is not a typical application of organohalides?

Using all organohalides as antibiotics. Signup and view all the answers

What happens during the reaction of alcohols with thionyl chloride?

Alcohols form alkyl chlorides in the presence of pyridine. Signup and view all the answers

What is a common result when heating an alkyl bromide with sodium iodide in acetone?

Formation of alkyl iodides via halogen exchange. Signup and view all the answers

Study Notes

Organic Halides

Alkyl halides are also known as haloalkanes.
They are compounds where one or more hydrogen atoms in an alkane are replaced by halogen atoms (fluorine, chlorine, bromine or iodine).
Halogen atoms are more electronegative than carbon atoms, leading to a polarized C-Hal bond.
Alkyl halides are classified based on the position of the halogen atom on the carbon chain.
Different types of alkyl halides exhibit differences in chemical properties.
Various alkyl halides exist as gases, liquids or solids at room temperature, depending on their size.
They are insoluble in water; they are soluble in organic solvents.
Alkyl bromides and iodides are denser than water; alkyl chlorides and fluorides are lighter than water.
Electronegativity decreases in the order of F > Cl > Br > I
Carbon-halogen bond lengths increase in the order of C-F < C-Cl < C-Br < C-I
Bond dipole moments decrease in the order of μ (C-Cl) > 1.56D > μ(C-F) > 1.51D > μ(C-Br) > 1.48D > μ(C-I) > 1.29D
Alkyl halides have higher boiling and melting points than alkanes with the same carbon number.
Boiling points and melting points increase with the size of R and X.

Introduction

Alkyl halides are categorized based on the hybridization of the carbon atom bonded to the halogen atom.
Alkyl: sp³ hybridized carbon.
Vinyl: sp² hybridized carbon in an alkene.
Aryl: sp² hybridized carbon in an aromatic ring.
Benzylic: sp³ hybridized carbon bonded to an aromatic ring.

Classification of Alkyl Halides

Based on the number of halogen atoms present:
- Monoalkyhalide: one halogen atom.
- Dihalide: two halogen atoms
- Trihalide: three halogen atoms.
Based on the position of the halogen atom along the carbon chain:
- Primary (1°): halogen is bonded to a primary carbon atom.
- Secondary (2°): halogen is bonded to a secondary carbon atom.
- Tertiary (3°): halogen is bonded to a tertiary carbon atom.
Geminal dihalide: two halogen atoms bonded to the same carbon atom.
Vicinal dihalide: two halogen atoms bonded to adjacent carbon atoms.

Nomenclature of Alkyl Halides

Alkyl halides are named according to IUPAC rules.
The halogen is treated as a substituent, using prefixes like fluoro-, chloro-, bromo-, and iodo-.
Different types of alkyl halides have specific common names
CH₂X₂ type compounds are methylene halides.
CHX3 type compounds are haloforms.
CX₄ type compounds are carbon tetrahalides.

Uses of Organohalides

Solvents: Industrial and household solvents (carbon tetrachloride, methylene chloride).
Reagents: Used as starting materials in organic synthesis, nucleophilic reactions, and elimination reactions.
Refrigerants: Freons (ChloroFluoroCarbons).
Pesticides: DDT, Aldrin, Chlordan.
Food Industry: Artificial sweeteners (e.g., sucralose)

Preparation of Alkyl Halides

Halogenation of Alkanes: Alkanes react with Cl₂ or Br₂ (in presence of UV light or high temperatures) to produce alkyl halides and polyhalogenated derivatives.
Allylic Brominations: Cyclohexene produces a high yield of 3-bromocyclohexene, by substitution of an allylic hydrogen for bromine
Action of Halogen Acids on Alcohols: Alcohols react with HBr or HI to form alkyl bromides/iodides. Dry HCl in presence of zinc chloride catalyzes the production of alkyl chlorides.
Action of Thionyl chloride on Alcohols: Alcohols react with thionyl chloride (SOCl₂) in the presence of pyridine to yield alkyl chlorides. Pyridine absorbs the formed hydrogen chloride.
Halogen Exchange reactions: alkyl bromides/chlorides can be converted to alkyl iodides under conditions of concentrated sodium iodide in acetone.
Alkyl fluorides can be prepared by treating alkyl chlorides/bromides with inorganic fluorides.

Chemical Properties of Organic Halogen Compounds

The main classes of reactions are nucleophilic substitution, elimination, and reduction.
Nucleophilic substitution reactions: one part (halogen) of the original compound is replaced by another, typically a nucleophile.
Elimination reactions: two groups are removed, often producing a pi bond.
Reduction reactions: occur when alkyl halide reacts with a suitable reducing agent, producing an alkane

Nucleophilic Substitution Reactions

Electron-deficient carbon atom in alkyl halides makes them react with electron-rich nucleophiles.
Nucleophile: species with unshared electron pair (lone-pair electrons), typically attacking the alkyl halide from behind the halogen group
Mechanisms of substitution reactions: SN1 and SN2
Common nucleophilic reagents (negative ions): H⁻, HS⁻, R⁻O⁻, CN⁻, halide ions, ...etc.
Common types of nucleophilic reagents (neutral molecules): H₂O, R-OH, R-OR, R3N, ...etc.

Substitution, Nucleophilic, Bimolecular: The SN2 Reaction

Nucleophile attacks the carbon bearing the leaving group, from the back side.
Electron pair of the nucleophile overlaps with the empty antibonding orbital of the carbon bearing the leaving group.
The bond between nucleophile and carbon is forming; bond between carbon-leaving group is breaking
Inversion of configuration
Rate law is second order, depending on both reactants' concentration.

Substitution Nucleophilic, Unimolecular: The SN1 Reaction

Two steps involved
Step 1: Reversible ionization of the alkyl halide, generating a carbocation intermediate.
Step 2: Nucleophilic attack on the carbocation intermediate.
Rate law follows first-order kinetics (dependent on the concentration of the alkyl halide.
Racemization occurs.

Factors Affecting the Rate of the SN2 Reaction

Substrate structure.
Concentration and reactivity of the nucleophile.
Effect of the solvent.
Nature of the leaving group. Bulkier the R groups around the carbon bonded to the leaving group, the slower is the SN2 reaction. More polar aprotic solvents favor SN2 reactions. Poor leaving groups will result in slower reaction rates for SN2.

Factors Affecting the Rate of the SN1 Reaction

Substrate structure
Effect of the solvent
Nature of the leaving group.
A more substituted alkyl group will result in a faster SN1 reaction
Polar protic solvents will favor SN1 reactions.

Elimination Reactions

Removal of two groups from a molecule, typically resulting in the formation of a new π bond.
Two common mechanisms: E1 and E2
E1 reaction: Unimolecular elimination in two steps.
Step 1: Ionization of the alkyl halide to form a carbocation.
Step2: Deprotonation/removal of a hydrogen from the adjacent carbon resulting in the alkene product.
E2 reaction: Bimolecular elimination reaction in one step.
Base removes a proton from a beta carbon, while the halogen leaves from the alpha carbon

Factors Affecting the Rate of E1 and E2 Reactions

Structure of the alkyl halide; higher substitution leads to faster reactions in both cases.
Base strength; stronger bases favor E2.
Leaving group ability; better leaving group favors both eliminations.
Solvent; polar aprotic solvents favor E2.

Reactions of Organometallic Compounds

Alkali metals (Li, Na, K, etc.) and alkaline earth metals (Mg, Ca, Zn) are good reducing agents.
Useful for forming Grignard reagents (R-MgX) and other organometallic compounds via reduction of alkyl halides.

Stereochemistry of SN1 and SN2 Reactions

SN2 reactions typically exhibit inversion of configuration.
SN1 reactions typically show racemization (retention and inversion of configuration). The products are a racemic mixture, which is a 50/50 mixture.

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Description

This quiz explores the fascinating world of alkyl halides, also known as haloalkanes. Learn about their structure, properties, classifications, and the influence of halogen atoms on their behavior. Test your understanding of these important organic compounds!