Organic Chemistry: Alkyl Halides

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Questions and Answers

Which type of alkyl halide generally exhibits higher boiling points due to increased surface area?

Linear alkyl halides (correct)

Polyhalogenated alkyl halides

Branched alkyl halides

Acyclic alkyl halides

What is the boiling point of tert-butyl bromide compared to n-butyl bromide?

Equal to the boiling point of n-butyl bromide

Lower than n-butyl bromide (correct)

Cannot be determined

Higher than n-butyl bromide

Which factor has a smaller impact on the boiling points of alkyl halides?

Dipole-dipole attractions (correct)

Hydrogen bonding

Molecular weight

Surface area

Which alkyl halides are generally denser than water?

Alkyl chlorides with two or more chlorine atoms Signup and view all the answers

When water is shaken with hexane, which compound is found in the top phase?

Hexane Signup and view all the answers

Why is free-radical halogenation not generally an effective method for synthesizing alkyl halides?

It often results in a mixture of products Signup and view all the answers

What is a benefit of using free-radical halogenation in industrial synthesis of alkyl halides?

Separation via simple distillation Signup and view all the answers

Which of the following will occur when water is shaken with chloroform?

Water will be in the top phase Signup and view all the answers

What is the primary reason chloroform was abandoned as an anesthetic?

It is toxic and carcinogenic. Signup and view all the answers

Which anesthetic is often used topically for minor procedures due to its cooling effect?

Ethyl chloride Signup and view all the answers

What environmental concern is associated with the release of freons into the atmosphere?

Catalyzing ozone decomposition. Signup and view all the answers

What was Freon-12 primarily replaced with in aerosol cans?

Carbon dioxide or low-boiling hydrocarbons Signup and view all the answers

Which halogenated anesthetic is less toxic than chloroform and goes by the trade name halothane?

CF3CHClBr Signup and view all the answers

Which of the following substances is considered an HCFC?

Freon-22 Signup and view all the answers

What impact did alkyl halides have on human health historically?

They were effective as insecticides. Signup and view all the answers

What historical event caused a significant population decline in Europe due to insect-borne diseases?

The Black Death Signup and view all the answers

In free-radical halogenation of cyclohexane, what is controlled by using a small amount of chlorine and an excess of cyclohexane?

Side reactions leading to dichlorides and trichlorides Signup and view all the answers

What makes free-radical bromination highly selective?

The formation of allylic radicals Signup and view all the answers

Which hydrogen atoms are preferentially abstracted in the free-radical bromination of isobutane?

3o hydrogen atoms Signup and view all the answers

What type of radical is formed when a bromine radical abstracts an allylic hydrogen atom?

Resonance-stabilized allylic radical Signup and view all the answers

What occurs after the formation of a resonance-stabilized allylic radical in the mechanism of bromination?

The allylic radical reacts with a bromine molecule Signup and view all the answers

Which characteristic of the allylic radical contributes to its stability compared to typical radicals?

Lower energy requirement for formation Signup and view all the answers

What is the result of the second resonance form of the allylic radical reacting with bromine?

Allylic shift product Signup and view all the answers

During the free-radical bromination of cyclohexene, where does the bromine atom usually substitute?

On the carbon adjacent to the double bond containing an allylic hydrogen Signup and view all the answers

Why are small anions more strongly solvated than large anions in a protic solvent?

Solvent molecules can approach small anions more closely. Signup and view all the answers

What effect does enhanced solvation have on the nucleophilicity of small anions?

It decreases their nucleophilicity. Signup and view all the answers

How does the polarizability of anion species vary in relation to atomic number?

Polarizability increases with increasing atomic number. Signup and view all the answers

In what type of solvent do anions typically exhibit increased reactivity?

Aprotic solvents Signup and view all the answers

What is one disadvantage of using aprotic solvents in reactions?

They do not enhance nucleophilicity. Signup and view all the answers

What role do crown ethers play in reactions involving cations and anions?

They enhance the nucleophilic strength of anions by solvating cations. Signup and view all the answers

Which condition is necessary for a molecule to be a suitable substrate for SN2 reactions?

It must have a single electrophilic carbon atom. Signup and view all the answers

What happens to fluoride ion's nucleophilicity in aprotic solvent compared to protic solvent?

It is more nucleophilic in aprotic solvents. Signup and view all the answers

What is the process called when an asymmetric carbon atom's configuration is inverted?

Walden inversion Signup and view all the answers

In the reaction of cis-1-bromo-3-methylcyclopentane with hydroxide ion, what type of product is formed?

trans-3-methylcyclopentanol Signup and view all the answers

What characterizes a stereospecific reaction?

It involves different stereoisomers producing different stereoisomers of the product. Signup and view all the answers

In the SN1 reaction of tert-butyl bromide in boiling methanol, what acts as the nucleophile?

Methanol Signup and view all the answers

Which statement is true regarding the rate of the SN1 reaction involving tert-butyl bromide?

It is first order overall and depends only on the concentration of tert-butyl bromide. Signup and view all the answers

Why does the nucleophile not impact the rate of the SN1 reaction?

It is not present in the transition state of the rate-limiting step. Signup and view all the answers

What type of reaction is characterized by the nucleophile replacing a leaving group in an alkyl halide without involving the nucleophile during the slow step?

SN1 reaction Signup and view all the answers

What is the result of the reaction between (S)-1-bromo-1-fluoroethane and sodium methoxide?

Inversion of configuration Signup and view all the answers

What type of mechanism do tertiary alkyl halides typically undergo?

SN1 mechanism Signup and view all the answers

Which type of nucleophile is required for the SN2 reaction?

Strong nucleophile Signup and view all the answers

In what type of solvent does the SN1 reaction typically occur most effectively?

Very polar ionizing solvents Signup and view all the answers

How does the rate of an SN1 reaction relate to its reactants?

Proportional only to the alkyl halide's concentration Signup and view all the answers

Which substrates are unlikely to undergo SN1 substitutions?

Methyl halides Signup and view all the answers

What is the relationship between substrate structure and the mechanism used in nucleophilic substitution?

Secondary substrates can undergo either SN1 or SN2 Signup and view all the answers

Why might polar aprotic solvents enhance the strength of weak nucleophiles in the SN2 reaction?

They do not solvate nucleophiles strongly Signup and view all the answers

What effect does the strength of a nucleophile have on the rate of an SN2 reaction?

Strong nucleophiles are required for maximum reaction rate Signup and view all the answers

Study Notes

Chapter 6: Alkyl Halides

Alkyl halides are a class of halogenated organic compounds
Three major classes: alkyl, vinyl, and aryl halides
Alkyl halides: halogen bonded to sp³ carbon
Vinyl halides: halogen bonded to sp² carbon
Aryl halides: halogen bonded to sp² carbon
Physical properties and common uses of each vary

Learning Outcomes

Name alkyl halides, explain their physical properties, and describe their common uses.
Predict the products of substitution and elimination reactions, and explain what factors favor each type of reaction.
Identify the differences between first-order and second-order substitution and elimination reactions, and explain what factors determine the order of the reaction.
Given a set of reaction conditions, identify the possible mechanisms, and predict which mechanism(s) and product(s) are most likely.

Reactivity of Alkyl Halides

The carbon-halogen bond is polar, making the carbon atom electrophilic.
A nucleophile can attack the electrophilic carbon.
The halogen atom can leave as a halide ion.
Alkyl halides can be substituted or eliminated by various reactions.

Nomenclature of Alkyl Halides

IUPAC nomenclature: names are based on the longest carbon chain & substituents (fluoro-, chloro-, bromo-, iodo-).
Common names are used for simpler alkyl halides (e.g., isopropyl bromide).

Classification of Alkyl Halides

Primary (1°): halogen bonded to one carbon
Secondary (2°): halogen bonded to two carbons
Tertiary (3°): halogen bonded to three carbons
Methyl halide: halogen bonded to a methyl group

Dihalides (with examples)

Geminal: two halogens on the same carbon
Vicinal: two halogens on adjacent carbons

Common Uses of Alkyl Halides

Solvents: carbon tetrachloride (dry cleaning), chloroform, methylene chloride
Refrigerants: Freons (CFCs)
Anaesthetics: chloroform, halothane
Pesticides: DDT, other chlorinated insecticides
Many synthetic uses as starting materials

Densities of Alkyl Halides

Alkyl fluorides and chlorides are less dense than water.
Alkyl bromides and iodides are denser than water.

Free-Radical Halogenation

Not an effective method for alkyl halide synthesis.
Usually produces many different products due to different reactivity levels of hydrogen atoms.

SN2 Reaction

Nucleophile replaces the halogen.
The halogen leaves with its electrons as a halide ion.
The reaction takes place in one step.
The rate depends on both the alkyl halide concentration and nucleophile concentration.
Inverted configuration, with the nucleophile attacking from the backside of the carbon.

SN1 Reaction

Ionization forms a carbocation prior to the nucleophile attack.
Rate depends on the carbocation stability and substrate concentration.
The carbocation is planar.
There's a mixture of possible products: retention and inversion of configuration

E1 Reaction

The reaction forms a carbocation intermediate, leading to an eventual alkene product.
The reaction rate is slower for secondary and primary alkyl halides.
A weak base often serves as the attacking reagent during the second step.
A possible method to determine if the reaction will result in an E1 product or a SN1 product if there's a difference in leaving groups or if the base for the reaction is strong enough

E2 Reaction

A strong base removes a proton.
The base and the substrate are in a transition state.
Formation of a new double bond during this reaction.
The reaction rate is faster than SN1 and E1
A concerted reaction: bonds break and form simultaneously.

Factors Affecting SN1/SN2/E1/E2 Reactions

Substrate: Methyl > Primary > Secondary > Tertiary.
Nucleophile: Strong nucleophiles favor SN2; weak nucleophiles favor SN1.
Base: Strong bases favor E2; weak bases favor E1.
Solvent: Polar protic solvents favor SN1 and E1; polar aprotic solvents favor SN2 and E2.
Stereochemistry: SN2 results in inversion, while SN1 and El produce a racemic mixture.

Summary

The reactions of alkyl halides (substitution and elimination) are important in organic chemistry
The rate, mechanism, stereochemistry, and product types depend on different factors which this chapter details.

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Description

Test your knowledge on alkyl halides, including their boiling points, density characteristics, and uses in anesthesia. This quiz covers factors affecting boiling points, free-radical halogenation, and environmental concerns related to halogenated compounds. Perfect for students studying organic chemistry concepts.