Organic Chemistry Chapter 10: Organohalides Reactions

What is an alkyl halide?

An organic compound containing at least one carbon-halogen bond (C-X), where X can be F, Cl, Br, or I.

What is the reactivity order of alkanes toward halogenation?

R3C· > R2CH· > RCH2·

How can alkyl halides be prepared from alkenes?

By reaction with N-bromosuccinimide (NBS) to give the product of allylic bromination.

What is the role of the allylic radical in NBS bromination?

The allylic radical is stabilized by resonance during the bromination process.

Which alcohols react well to form alkyl halides?

Tertiary alcohols (R3COH) react well; primary and secondary alcohols require reagents like SOCl2 or PBr3.

What are Grignard reagents?

Organomagnesium halides (RMgX) formed when alkyl halides react with magnesium in ether solution.

What is the function of Gilman reagents?

Diorganocoppers (LiR2Cu) that react with organohalides to yield coupled hydrocarbon products.

What is the significance of organohalides in organic chemistry?

They undergo nucleophilic substitutions and eliminations, serving as a model for more complex reactions.

What are some properties and uses of organohalides?

They are used as fire-resistant solvents, refrigerants, and pharmaceuticals.

What is the process of radical halogenation?

A method to prepare alkyl halides from alkanes, typically resulting in a mixture of products.

What is the allylic position in organic compounds?

The carbon atom adjacent to a double bond in an alkene.

What reagents are commonly used for converting alcohols to alkyl halides?

SOCl2, PBr3, or HF in pyridine.

What is a carbanion?

A negatively charged carbon species that can act as a nucleophile in organic reactions.

What is the main focus of Chapter 10 in organic chemistry?

To study the naming, preparation, and reactions of alkyl halides.

What is the general formula for organohalides?

C-X, where C is carbon and X is a halogen (F, Cl, Br, I).

What type of reactions do alkyl halides typically undergo?

Nucleophilic substitutions and eliminations.

What is the significance of resonance in allylic radicals?

Resonance stabilizes the allylic radical, facilitating reactions like bromination.

What is the outcome of the reaction between alkyl halides and magnesium?

Formation of Grignard reagents (RMgX).

What is the role of CuI in reactions involving Gilman reagents?

It facilitates the formation of diorganocopper compounds from organolithium reagents.

What is the primary challenge in preparing alkyl halides from alkanes?

The process often yields a mixture of products due to radical halogenation.

What is the mechanism of allylic bromination?

Involves the formation of an allylic radical that is stabilized by resonance.

How do alkyl halides react with nucleophiles?

They undergo nucleophilic substitution reactions, where the halogen is replaced by the nucleophile.

What is the first step in naming alkyl halides?

Find the longest chain and name it as the parent chain.

How do you determine the numbering of the carbon chain in alkyl halides?

Number from the end nearest any substituent (alkyl or halogen).

What is the effect of the periodic table position on the C-X bond?

The C-X bond is longer and weaker as you go down the periodic table.

What type of product is formed when HCl, HBr, or HI is added to alkenes?

An alkyl halide is formed, typically as a Markovnikov product.

What is radical halogenation?

A reaction where alkanes react with Cl2 or Br2 in the presence of heat or light, replacing C-H with C-X.

What is a significant drawback of radical halogenation?

It produces mixtures and is hard to control.

What does the relative reactivity of halogenation depend on?

It parallels the stability of radicals formed during the reaction.

What is N-bromosuccinimide (NBS) used for?

It selectively brominates allylic positions and requires light for activation.

Why is the allylic radical more stable than a typical alkyl radical?

The allylic radical is delocalized and more stable by 40 kJ/mol (9 kcal/mol).

What is the outcome of allylic bromination with NBS?

It creates an allylic bromide, which can react with a base to produce a conjugated diene.

How can alkyl halides be prepared from alcohols?

By reacting tertiary alcohols with HX (HCl or HBr) in an ether solution.

What is the product of the reaction of RX with Mg in ether or THF?

An organometallic compound (alkyl-metal bond) RMgX is formed.

What is formed when alkyllithium (RLi) reacts with copper iodide?

Lithium dialkylcopper (Gilman reagents) is formed.

What is the significance of organometallic coupling reactions?

They produce larger molecules of defined structure from two organometallic molecules.

What defines oxidation in organic chemistry?

Oxidation occurs when a carbon or hydrogen connected to a carbon is replaced by oxygen, nitrogen, or halogen.

What is the opposite of oxidation in organic chemistry?

Reduction, which results in a gain of electron density at carbon.

What is the result of a reduction reaction?

Formation of C-H (or C-C) bonds and breaking of C-O, C-N, C-X bonds.

What types of reactions do alkyl halides undergo?

Nucleophilic substitutions and eliminations.

What are organohalides?

Compounds that contain one or more halogen atoms bonded to carbon.

What is a common use of chloromethane?

It is released by ocean kelp and used in various industrial applications.

What is the significance of allylic stabilization?

It allows for the formation of more stable radicals during reactions.

What happens during the reaction of tertiary C-OH with HX?

It is fast and effective, leading to the formation of alkyl halides.

What is the role of light in radical halogenation?

Light is required to initiate the radical mechanism for halogenation.

What is the characteristic of the C-X bond in alkyl halides?

It is polarized, with a slight positive charge on carbon and a slight negative charge on halogen.

What is the typical structure of alkyl halides?

Compounds with a halogen atom bonded to a saturated, sp3-hybridized carbon atom.

What are alkyl halides systematically named as?

Haloalkanes

How should the carbons of the parent chain be numbered?

Beginning at the end nearer the first substituent, whether alkyl or halo.

What should be done if different halogens are present in an alkyl halide?

Number each halogen and list them in alphabetical order when writing the name.

What is an example of a common name for CH3I?

Iodomethane or methyl iodide.

How do bond lengths and strengths of carbon-halogen bonds change down the periodic table?

Bond lengths increase and bond strengths decrease.

What is the dipole moment of CH3F?

1.85 D

What is the bond length of CH3Cl?

178 pm

What is the bond strength of CH3Br?

294 kJ/mol (70 kcal/mol)

What is the significance of the C−X bond polarity in alkyl halides?

The carbon atom bears a slight positive charge (δ+) and the halogen a slight negative charge (δ−), making the carbon atom behave as an electrophile in polar reactions.

What is the first step in the radical halogenation of alkanes?

Initiation, where radicals are produced.

What are the three kinds of steps required in radical substitution reactions?

Initiation, propagation, and termination.

Why is alkane halogenation considered a poor synthetic method?

It results in mixtures of products.

What is the reactivity order of hydrogen atoms toward chlorination?

Tertiary > Secondary > Primary.

How does the stability of radicals relate to bond dissociation energies?

Weaker C−H bonds correspond to more stable radicals.

What is the product of radical chlorination of butane?

A mixture of monochlorinated products, including 1-chlorobutane and 2-chlorobutane.

What is allylic bromination?

The substitution of hydrogen by bromine at the position next to the double bond using N-bromosuccinimide (NBS).

What is the product of allylic bromination of cyclohexene?

3-bromocyclohexene.

What type of mechanism do hydrogen halides use to react with alkenes?

A polar mechanism resulting in Markovnikov addition.

What are the products of bromine and chlorine reactions with alkenes?

1,2-dihalogenated products through anti addition.

What is the role of ultraviolet light in radical halogenation?

It initiates the radical reaction process.

What happens during the termination step of radical substitution?

Two radicals combine to stop the chain reaction.

What is the expected outcome of monochlorination of 2-methylbutane?

A mixture of products with varying percentages based on hydrogen reactivity.

What is the approximate percentage of 1-chlorobutane produced from butane chlorination?

30%.

What is the approximate percentage of 2-chlorobutane produced from butane chlorination?

70%.

How does the number of hydrogen types affect the products in alkane chlorination?

Different types of hydrogen lead to different product ratios based on their reactivity.

What initiates the allylic bromination process?

A Br· radical abstracts an allylic hydrogen atom.

What is formed when a Br· radical abstracts an allylic hydrogen?

An allylic radical plus HBr.

How does HBr contribute to the bromination process?

HBr reacts with NBS to form Br2.

What is the result of Br2 reacting with an allylic radical?

It yields the brominated product and regenerates a Br· radical.

Why does bromination with NBS occur exclusively at the allylic position?

Due to the relative stabilities of various radicals, particularly the allylic radical.

What is the bond strength of an allylic C−H bond?

About 370 kJ/mol (88 kcal/mol).

How does the stability of an allylic radical compare to a typical alkyl radical?

An allylic radical is more stable by about 40 kJ/mol (9 kcal/mol).

What hybridization does the radical carbon in an allylic radical adopt?

sp2 hybridization.

What does the electronic symmetry of the allylic radical allow?

It allows for two equivalent resonance structures.

What is the significance of resonance forms for the stability of the allylic radical?

The greater the number of resonance forms, the greater the stability of the compound.

What happens during the reaction of Br2 with an allylic radical?

The reaction can occur at either end of the radical, leading to a mixture of products.

What is a common product of allylic bromination of unsymmetrical alkenes?

A mixture of products, such as 3-bromo-1-octene and 1-bromo-2-octene.

What method is commonly used to prepare alkyl halides from alcohols?

Treating the alcohol with HCl, HBr, or HI.

Which type of alcohol reacts fastest with HX to form alkyl halides?

Tertiary alcohols (R3COH).

What reagents are typically used to convert primary and secondary alcohols to alkyl halides?

Thionyl chloride (SOCl2) or phosphorus tribromide (PBr3).

What is the advantage of using SOCl2 or PBr3 over HX for alcohol conversion?

They are less acidic and less likely to cause rearrangements.

What is a characteristic of alkyl fluorides preparation from alcohols?

Numerous alternative reagents like diethylaminosulfur trifluoride and HF in pyridine are used.

What type of bonds do Grignard reagents contain?

Carbon-metal bonds.

Who discovered Grignard reagents?

Francois Auguste Victor Grignard.

What is the significance of Grignard reagents in organic chemistry?

They are used for various reactions involving organometallic compounds.

What happens to the unpaired electron in an allylic radical?

It is delocalized over an extended π-orbital network.

What is the effect of delocalization on the reactivity of the allylic radical?

It allows for reactions to occur at either end of the π orbital system.

What is the expected product when 4,4-dimethylcyclohexene reacts with NBS?

A mixture of up to four products due to different allylic positions.

What is the role of resonance in the stability of the allyl radical?

Resonance forms increase stability by spreading out electron density.

How does the reaction of alcohols with HX typically proceed?

By bubbling pure HX gas into a cold ether solution of the alcohol.

What types of halides can Grignard reagents be made from?

Alkenyl (vinylic) and aryl (aromatic) halides.

Which halogens can be used in the formation of Grignard reagents?

Chlorine (Cl), Bromine (Br), and Iodine (I).

Why are chlorides less reactive than bromides and iodides in Grignard reactions?

Chlorides are less reactive due to their stronger carbon-halogen bond.