第七章卤代烃 halohydrocarbon

Halohydrocarbon :

Classification of Halogen-Substituted Hydrocarbons (TO)Nomenclature of Halogen-Substituted Hydrocarbons (TO)Preparation of Halogen-Substituted Hydrocarbons (TO)Structure of Halogen-Substituted Hydrocarbons (TO)Chemical Properties of Halogen-Substituted Hydrocarbons (TO)

Teaching contents

The compound that one (or more) hydrogen atom in a hydro-carbon is substituted by halogen atom is called halogen-substituted hydrocarbon.According to the hydrocarbon, halogen-substituted hydrocarbon can be classified into: Alkyl halogen, alkenyl halogen, alkynyl halogen, aryl halogen.According to the degree of carbon, halogen-substituted hydro-carbon can be classified into:

primary secondary tertiary halide halide halide Classification of Halogen-Substituted Hydrocarbons

Alkyl halides are named in the same way as alkanes, by treating the halogen as a substituent on a parent alkane chain. There are three rules:Rule 1 Find the longest carbon chain and name it as the parent. If a double or triple bond is present, the parent chain must contain it.Rule 2 Number the carbon atoms of the parent chain, beginning at the end nearer the first substituent, regardless of whether it is alkyl or halo. Assign each substituent a number to its position on the chain.Rule 3 If the parent chain can be properly numbered from either end by rule 2, begin at the end nearer the substituent (either alkyl or halo) that has alphabetical precedence. Nomenclature of Halogen-Substituted Hydrocarbons(1)

For example:

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BACK Preparation of Halogen-Substituted Hydrocarbons

Free radical substitution of alkane

Additions of small cycloalkanes

Addition of Halogens to Alkenes

Hydrohalogenation of Alkenes

Preparation of Halogen-Substituted Hydrocarbons (1)

Hydrohalogenation of Alkynes

Free Radical Substitution of Alkenes

Aromatic Halogenation

Bromination of Alkylbenzene Side Chains

Preparation of Halogen-Substituted Hydrocarbons (2)

Preparing Alkyl Halides from AlcoholsThe most general method for preparing alkyl halides is to make them from alcohols. The simplest method for converting an alcohol to an alkyl halide involves treating the alcohol with HCl, HBr, or HI.

Primary and secondary alcohols are best converted into alkly halides by treatment with such reagents as thionyl chloride (SOCl2) or phosphorus tribromide (PBr3).

BACK Preparation of Halogen-Substituted Hydrocarbons (3)

The structure of alkyl halidesThe carbon-halogen bond in an alkyl halide results from the overlap of a carbon sp3 orbital with a halogen orbital. Thus, alkyl halides carbon atoms have an approximately tetrahedral geometry. Since halogens are more electro-negative than carbon. The C-X bond is therefore polar, with the carbon atom bearing a slight positive charge (+) and the halogen a slight negative charge (-).

Structure of Halogen-Substituted Hydrocarbons (1)

The structure of vinylic halides and aryl halides p, -conjugated system in vinylic halides and aryl halides

BACK Structure of Halogen-Substituted Hydrocarbons (2)

Nucleophilic Substitutions of Alkyl Halides (TO)The SN2 Reaction and SN1 Reaction (TO)Elimination Reactions of Alkyl Halides (TO)Reactions of Halides: Grignard Reagents (TO)Organometallic Coupling Reactions (TO)Nucleophilic Aromatic Substitutions (TO)Benzyne---Elemination/Addition mechanism (TO)Reactions of Allyl Halides and Benzyl Halides (TO)

Chemical Properties of Halogen-Substituted Hydrocarbons (1)

Nucleophilic Substitutions of Alkyl HalidesAlkyl halides can undergo substitution of the X group by the nucleophile (Nu):


For example:

NEXT Chemical Properties of Halogen-Substituted Hydrocarbons (3)

The SN2 ReactionIn every chemical reaction, there is a direct relationship between reaction rate and reactant concentrations. When we measure this relationship, we measure the kinetics (of the reaction. For example, lets look at the kinetics of a simple nucleophilic substitution of CH3Br with OH- to yield CH3OH plus Br -.

This equation says that the rate of disappearance of reactant is equal to a constant of k times the alkyl halide concentration times the hydroxide ion concentration. So the rate of this reaction is dependent on the concentrations of two species, and the reaction is second-order reaction. Chemical Properties of Halogen-Substituted Hydrocarbons (4)

The SN2 ReactionThe essential feature of the SN2 mechanism is that the reaction takes place in a single step without intermediates. For example:

This reaction occurs through a transition state in which the new HO-C bond is partially forming at the same time that the old C-Br is partially breaking. The transition state for this inversion has the remaining three bonds to carbon in a planar arrangement. The stereochemistry at carbon is inverted. ( Walden Inversion, ) Chemical Properties of Halogen-Substituted Hydrocarbons (5)

The SN1 ReactionWe might expect that the reaction of a tertiary substrate (hindered) with water to be the slowest of substitution reactions. Remarkably, however, the opposite is true.

What happened? Clearly, these reactions cant be taking place by the SN2 mechanism we have been discussing. This alternative mechanism is called the SN1 reaction.


The reaction of (CH3)3CBr with H2O looks analogous to the reaction of CH3Br with OH-, and we might therefore expect to observe second-order kinetics. In fact, we do not.

We find instead that the reaction rate is dependent only on the alkyl halide concentration and is independent of the H2O concentration. In other words, the reaction is a first-order process.


The mechanism of the reaction of 2-bromo-2-methylpropane with H2O:


Stereochemistry of the SN1 Reaction:Since an SN1 reaction occurs through a carbocation intermediate, its stereochemical outcome should be different from that for an SN2 reaction. Since carbocations are planar and are achiral. The carbocation intermediate can be attacked by a nucleophile equally well from either side, leading to a 50:50 mixture of enantiomers a racemic mixture. For example:


The factors that effect the SN1 and SN2 reactionsChemical Properties of Halogen-Substituted Hydrocarbons (10) NEXT

Elimination Reactions of Alkyl HalidesWhen a nucleophile/Lewis base reacts with an alkyl halide, the nucleophile can attack at a neighboring hydrogen and cause elimination of HX to form an alkene.


Regiochemistry of Elimination of Alkyl Halides- Zaitsevs RuleWhat products result from loss of HX from an unsymmetrical halide? According to a rule of formulated in 1875 by the Russian chemist Alexander Zaitsev, base-induced elimination reactions generally give the more highly substituted (more stable) alkene product. For example:

The reaction gives mixtures of butene products because elimination reactions can take place through two different mechanistic pathways: the E1and E2 reactions.Chemical Properties of Halogen-Substituted Hydrocarbons (12)

The E2 ReactionThe E2 reaction (for elimination, bimolecular) occurs when an alkyl halide is treated with a strong base, such as hydroxide ion or alkoxide ion (RO-). It is the most commonly occurring pathway for elimination and can be formulated as shown below:

Like the SN2 reaction, the E2 reaction takes place in one step without intermediate. As the attacking base begins to abstract H+ from a carbon next the leaving group, the C-H bond begins to break, a C=C bond begins to form, and the leaving group begins to depart, taking with it the electron pair from the C-X bond.Chemical Properties of Halogen-Substituted Hydrocarbons (13)

The stereochemistry of E2 reactionAs shown by a large number of experiments, E2 reactions always occur with a periplanar geometry, meaning that all four reacting atoms-the hydrogen, the two carbons, and the leaving group- lie in the same plane. Two such geometries are possible: syn periplanar geometry, in which the H and X are on the same side of the molecule, and anti periplanar geometry, in which the H and the X are on opposite sides of the molecule. Of the two choices, anti periplanar geometry is energetically preferred because it allows the substituents on the two carbons to adopt a staggered relationship, whereas syn geometry requires that the substituents on carbon be eclipsed. For example:


The E1 ReactionJust as the E2 Reaction is analogous to the SN2 reaction, there is a close analog to the SN1 reaction called the E1 reaction (for elimination, unimolecular). The E1 reaction can be formulated as shown below:


The factors that effect the E1 and the E2 reactions

Chemical Properties of Halogen-Substituted Hydrocarbons (16) NEXT

Reactions of Halides: Grignard ReagentsOrganohalides, RX, react with magnesium metal in dry ether or tetrahydrofuran (THF) solvent to yield organo-magnesium halides, RMgX. The products, called Grignard reagents after their discoverer, Victor Grignard (1912 Nobel Prize winner), are examples of organo-metallic compounds because they contain a carbon-metal bond.


The reactions of Ethylmagnesium bromide:


Organometallic Coupling ReactionsMany other kinds of organometallic compounds can be prepared in a manner similar to that of Grignard reagents. For example, alkyllithium reagents, RLi, can be prepared by the reaction of an alkyl halides with lithium metal. For example:

One of the most valuable reactions of alkyllithiums is their use in making lithium diorganocopper compounds, R2CuLi, called Gilman reagents. For example:

Gilman reagents are useful because they undergo organometallic coupling reactions with alkyl halides to yield hydrocarbon product.


Nucleophilic Aromatic SubstitutionsAromatic substitution reactions usually occur by an electrophilic mechanism. Aryl halides that have electron-withdrawing substituents, however, can also undergo nucleophilic aromatic substitution. For example:

Nucleophilic substitutions on an aromatic ring proceed by the addition /elimination mechanism shown below:


Benzyne---Elimination/Addition mechanismHalobenzenes without electron-withdrawing substituents do not react with nucleophiles under most conditions. At high temperature and pressure, however, even chlorobenzene can be forced to react.

Mechanism of reaction:


Reactions of Allyl Halides and Benzyl Halides


第 七 章 卤代烃 halohydrocarbon

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第七章卤代烃 halohydrocarbon