Haloalkanes and Haloarenes NCERT Solutions || Haloalkane Structure

haloalkanes and haloarenes, In an alkane molecule, a compound in which a hydrogen atom is partially or completely replaced by a halogen atom is called a haloalkane, which is referred to as a haloalkane. Monohaloalkanes can be represented by RX.

Classification of Haloalkanes

Haloalkanes can be classified based on the carbon atom to which the halogen atom is attached. When a halogen atom is connected to a primary, secondary, or tertiary carbon atom, it is referred to as a primary, secondary, or tertiary haloalkane, respectively.

Example: CH 3 CH 2 CH 2 Cl 1-chloropropane (1 °)

It is divided into monohaloalkane, dihaloalkane, and polyhaloalkane according to the number of halogen atoms.

According to the type of halogen atom, it is divided into fluoroalkane, chloroalkane, bromoalkane and iodoalkane.

According to the difference of alkyl, it is divided into saturated haloalkane, unsaturated haloalkane, and halogenated aromatic hydrocarbon.

Nomenclature of Haloalkanes

Some simple and common haloalkanes are usually named by common nomenclatures, such as methyl chloride, isopropyl bromide, tert-butyl chloride and so on.

Common names: CHCl 3 (chloroform): CF2 Cl2 (freon).

System designation of haloalkanes:

(A) The longest carbon chain including the carbon atom to which a halogen atom is connected is selected as the main chain, and the number of carbon atoms in the main chain is referred to as “an alkane”.

(B) Both branched and halogen atoms are used as substituents.

(C) Write the name and position of the substituent before the main paraffin name to obtain the full name.

Elemental composition

Carbon, halogen (fluorine, chlorine, bromine, iodine), hydrogen

Note: Some haloalkanes do not contain hydrogen, such as carbon tetrachloride (CCl4)

Physical Properties

Of the halogenated alkanes (except fluorinated alkanes), only methyl chloride, ethyl chloride , vinyl chloride and methyl bromide are gases, and the rest are colorless liquids or solids. 

However, iodoalkanes and bromoalkanes, especially iodoalkanes, have a long-term coloration due to the decomposition of free iodine and bromine. Monohaloalkane has an unpleasant odor and its steam is toxic. 

They are insoluble in water but soluble in organic solvents such as diethyl ether, benzene, hydrocarbons, etc. Some halogenated alkanes are very good organic solvents, such as dichloromethane and carbon tetrachloride. In the haloalkane molecule, as the number of halogen atoms increases, the flammability of the compound decreases.

The boiling point of haloalkanes increases with the number of carbon atoms in the molecule. Haloalkanes with the same number of carbon atoms, the boiling point is: iodoalkane> bromoalkane> chloroalkane.

In isomers, the more branched chains the lower the boiling point. The relative density of monochloroalkane is less than 1, and the relative density of monoiodoalkane and bromoalkane is greater than 1. In the same series, the relative density of haloalkane decreases with the increase of the number of carbon atoms.

C-X bond

Dipole Moment

In the halogenated alkane molecule, since the electronegativity of the halogen atom is greater than that of carbon, the electron cloud of the C-X bond is biased toward the halogen atom, and the C-X bond becomes a polar covalent bond.

The dipole moments of some simple haloalkanes are shown in the table.

Dipole moments of some simple haloalkanes (C · m)
X (halogen atom) CH 3 X CH 2 X 2 CHX 3 CX 4
F 6.07 * 10 ^ -30
Cl 6.47 * 10 ^ -30 5.34 * 10 ^ -30 3.44 * 10 ^ -30 0
Br 5.97 * 10 ^ -30 4.84 * 10 ^ -30 3.40 * 10 ^ -30 0
I 5.47 * 10 ^ -30 3.80 * 10 ^ -30 3.35 * 10 ^ -30 0

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Structure of Haloalkane

In the haloalkane molecule, the sp3 hybrid orbital of the halogen and the sp3 hybrid orbital of the carbon overlap to form a zeta bond, and the C-X bond length is:

C—H C—C C—F C—Cl C—Br C—I

0.110 nm 0.154 nm 0.139 nm 0.176 nm 0.194 nm 0.214 nm

The C-F bond length is between the C-H and C-C bond lengths. Therefore, fluorine is larger than hydrogen and smaller than carbon, and can form long-chain fluorocarbons. Such as polytetrafluoroethylene. The Cl and Br atoms have a larger volume, and the atoms are crowded with each other after the formation of the chain, which has a large repulsive force. When forming a long chain molecule, the carbon chain is broken due to the mutual crowding and repulsion between the atoms. Br cannot form long-chain perchlorocarbons and bromocarbons.

Conformation: The C-C bond rotation energy barrier of a halogenated alkane with a halogen atom is 13.4 to 15.5 kJ / mol, and its size has little to do with the volume of halogen. In the 1,2-dichloroethane molecule, the C-C bond rotation energy barrier is 13.4 kJ / mol, and the molecule has two stable conformations, adjacent cross and opposite cross. 

In the gas phase, the cross-conformation is 5 kJ / mol more stable than the adjacent cross-conformation, while in the liquid phase the stability of the two conformations is close to equal. This is due to two forces in the molecule, one is the dipole-dipole interaction, and the other is the van der Waals attraction. 

In the adjacent cross-conformation, the two C—Cl bonds produce dipole repulsion, while there is van der Waals gravitational force between the two chlorine atoms. In the gas phase, the dipole repulsion is dominant, so the cross-conformation is relatively stable. In the liquid phase, the stability of the two conformations is nearly equal because the repulsion between dipoles is reduced by the solvent.

Chemical properties

In the haloalkane molecule, since the CX (X = Cl, Br, I) bond is a polar covalent bond, it is relatively easy to break, so that the haloalkane can undergo various reactions. CF bonds generally have considerable stability, and their reactions differ from other halogens.

Nucleophilic substitution reaction

Halogens in halogenated hydrocarbons may be replaced by other atoms or groups. In the reaction, the halogen leaves in the form of negative ions, and the substituted atoms or groups are some nucleophiles. Lack of carbon nucleophile to attack the electron-substituted product is formed – a nucleophilic substitution reaction, SN represents.

SN2 mechanism: For the hydrolysis of methyl bromide, the reaction is a synchronous process. The nucleophile attacks the central carbon atom from the back of the leaving group, and first forms a weak bond. At the same time, the bond between the leaving group and the carbon is weakened to a certain extent. The other three bonds on the carbon atom also gradually occur The change, from umbrella to plane, requires energy (activation energy). With the progress of the reaction, when the state of maximum energy, that is, the transition state, is reached, new bonds are formed, old bonds are broken, and the remaining three bonds on the carbon atom are changed from planar to umbrella-shaped. 

The whole process is like an umbrella flipping in a strong wind. When a reactant generates a transition state, it needs to absorb activation energy. The transition state is the highest point of potential energy. Once a transition state is formed, energy is released and a product is formed. 

Since the step of controlling the reaction rate is a bimolecule, two molecules are required to collide with each other, so the reaction is a nucleophilic substitution of the bimolecules, and it appears as a secondary reaction.

SN1 mechanism: The SN1 reaction proceeds in steps. The reactants first dissociate into carbocations and negatively charged leaving groups. The reaction requires energy (activation energy) to form C + intermediates. This is to control the reaction rate.

When the molecule is dissociated, C + immediately binds to the nucleophile to form a product, and this step is extremely fast. C-X bond dissociation require higher energy, when the energy reaches the highest point, forming a first transition state Ts1 [R & lt . 3 C … .. X-], then rapidly dissociates into C + Intermediate, C + and of Nu  Bonding also requires a certain amount of energy to form products through the [R 3 C… .Nu] transition state Ts2. Because the step that determines the reaction rate is the step with the highest potential for the transition state, that is, the dissociation of the C-X bond, this step involves only one molecule, so the reaction is called a single-molecule nucleophilic substitution reaction.

Stereochemistry of S N2 reaction:

It can be seen from the SN2 reaction mechanism that the nucleophile attacked from the back of the leaving group, and as a result, the configuration changed. Ingold et al. exchanged the photoactive 2-iodooctane with the radioisotope iodide ion in acetone and found that the rate of racemization was twice the rate of the exchange reaction, indicating that the configuration of the product had been transformed. Walden transformation. 

The reactant 2-iodooctane has the S configuration. After the S N2 reaction, the configuration is completely transformed into the R configuration, with the optical rotation directions opposite. The R and S configurations form a racemate, and the optical rotation cancels. The spin rate is twice the rate of the exchange reaction.

Stereochemical evidence supports the S N2 mechanism, a complete conversion from the configuration, indicating that the nucleophile attacks the central carbon atom from the back of the leaving group. Most of the nucleophilic substitution reactions belong to the S N2 mechanism, and a lot of experimental facts prove this. 

Therefore, the S N2 reaction is always accompanied by a configuration inversion, or a complete configuration transformation is often a sign of the S N2 reaction. Why does the nucleophile always attack from the back of the leaving group? This is because

① attack from the front will be repelled by the leaving group carrying electrons.

② attacks from the back can form a more stable transition state, reducing the activation energy of the reaction.

Stereochemistry of S N1 reaction:

In the S N1 reaction, the formation of C + ions is a step that determines the overall reaction rate. C + ions are sp2 hybrid planar structures, and positively charged carbon atoms have an empty p orbital. When a nucleophile reacts with a C + ion, it can enter from both sides of the C + plane, and the reaction probability is equal. 

That is 50% configuration retention and 50% configuration conversion. The completely ideal S N1 reaction is only a limit case. In most cases, the product is not completely racemized, but it is often a part of racemization and a part of the configuration conversion. 

For example, (R) -2-bromooctane is hydrolyzed in an alkaline aqueous solution to obtain 83% of the configuration conversion product and 17% of the configuration retention product, that is, 34% of racemization occurs. To explain the partial racemization in this S N1 reaction, Weinstein.S explained the mechanism of ion pairing.

1. Hydrolysis: When co-heated with an aqueous solution of a strong base, the halogen atom is replaced by a hydroxyl group (-OH) to generate alcohol.

2. Interaction with sodium alkoxide: react with sodium alkoxide in the corresponding alcohol solution. The halogen atom is replaced by an alkoxy group (-OR) to form an ether, which is called the Williamson synthesis method. The tertiary haloalkanes are mainly eliminated olefins.

3. Interaction with sodium cyanide or potassium cyanide: the halogen atom is replaced by a nitrile group to generate a nitrile (R-CN), and the tertiary haloalkane mainly obtains the elimination product olefin.

4. Interaction with ammonia: Interaction with ammonia, halogen atoms are replaced by amino ( -NH 2 ) to form primary amines.

5. Interaction with silver nitrate: Interaction with silver nitrate in ethanol solution to generate silver halide precipitation.

The order of activity of different haloalkanes is: RI> RBr> RCl; when the halogen atoms are the same but the hydrocarbyl structure is different, the order of activity is: 3 °> 2 °> 1 °.

Eliminate (de) react

The haloalkane is co-heated with an alcohol solution of a strong base, and the elimination reaction mainly occurs, and a portion of the hydrogen halide is removed to generate an olefin. When a haloalkane is dehydrohalogenated, hydrogen atoms are mainly removed from adjacent carbon atoms that contain less hydrogen. This is an empirical rule called the Saytzeff rule.

Elimination mechanism (E1 and E2), the alkyl structure of the haloalkane is different, and the reaction proceeds according to different mechanisms.

E1: For tertiary bromobutane, the reaction is carried out in two steps. First, carbocations are generated, and then dehydrogenation is eliminated or added to produce substituted products.

The rate-determining step of the reaction is the formation of carbocations, and the reaction rate is only related to the concentration of the reactants, v = k [RX], which is a first-order reaction in kinetics and is a single-molecule reaction, which is represented by E1. It can be seen that E1 and S N1 both react through the same carbocation, therefore, in the course of the reaction, the two reactions compete with each other. 

Generally, high temperature is beneficial for elimination, because the elimination of protons to generate olefins requires higher activation energy; but in the presence of polar solvents and no strong bases, S N1 reacts quickly and the products are stable, mainly replacing products.

E2: If a base is present in the reaction system, as the concentration of the base increases, the elimination products increase. This shows that the reaction rate is not only related to the concentration of alkyl halide, and also on the concentration of the base, showing two reactions kinetically, the reaction is a bimolecular process, V = K [the RX] [the RO  ], represented by E2.

In the E2 reaction, the base attacks the β-H of the halogenated hydrocarbon, and bromine leaves with a pair of electrons to form a transition state. The formation of a new bond and the breaking of the old bond occur simultaneously. The reaction rate depends on the concentration of the reactants and the concentration of the base.

Comparing S N2 and E2, in the reaction of SN2, the base attacks the central carbon atom, and the strong nucleophilicity of the reagent is conducive to the SN2 reaction; while in the E2 reaction, the base attacks the number of β-H, β-H, and the acidity. Strong, strong alkaline is good for E2 reaction. Therefore, S N2 and E2 often occur simultaneously.

React with metals

1. Reacts with magnesium: reacts with metallic magnesium in anhydrous ether to form alkyl magnesium halides. Alkyl magnesium halide is also known as a Grignard reagent

In addition to the solvent ether, tetrahydrofuran, benzene, and other ethers are used in the preparation of this reagent. Ether and tetrahydrofuran are the best. This reagent reacts with carbon dioxide to form a carboxylic acid that is increased by one carbon. In the air, it is easily oxidized and hydrolyzed to form alcohol, it can be decomposed into hydrocarbons.

2. Reaction with lithium: In an inert solvent (such as diethyl ether, petroleum ether, etc.), it reacts with metallic lithium to form organolithium compounds. 

The compound reacts with cuprous halide to form an important organic synthesis reagent, dialkyl copper lithium, which is called an organic copper lithium reagent. This dialkyl copper lithium reagent reacts with a haloalkane to form an alkane, which is called Corey-House synthesis.

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