ALKYL HALIDES, Summaries of Chemistry

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CHAPTER

10 ALKYL HALIDES

Animation 10.1: Alkyl Halides reaction Source & Credit: chemwiki

10.1 INTRODUCTION

1. How to name an alkyl halide and to classify it into primary, secondary and tertiary alkyl halides.
2. Simple ways of generating alkyl halides.
3. The reason why C-X bond in chemistry is one of the most reactive type.
4. The general mechanistic details of nucleophilic substitution and elimination reactions.
5. The preparation of Grignard’s reagent, the reactivity of C-Mg bond and its synthetic applications in organic chemistry.

IN THIS CHAPTER YOU WILL LEARN:

Halogen derivatives of alkanes are called haloalkanes. They may be mono, di, tn or poly haloalkanes depending upon the number of halogen atoms present in the molecule.

Among these, monohaloalkanes are also called Alkyl Halides. Their general formula is R -- X , where R may be methyl, ethyl, propyl, etc. and X represents halogen atoms (F, Cl, Br, I). Mono haloalkanes or alkyl halides are further classified into primary, secondary and tertiary alkyl halides depending upon the type of carbon atom bearing the halogen atom.

In a primary alkyl halide halogen atom is attached with a carbon which is further attached to one or no carbon atom e.g.,

CH 3 CI

Chloromethane

CH 3 CH 2 CI

Chloroethane

CH 3 CH 2 CH 2 CI

1-Choropropane

Secondary alkyl halides are those in which halogen atom is attached with a carbon atom which is further attached to two other carbon atoms directly, e.g., secondary carbon atom.

In tertiary alkyl halides halogen atom is attached to a carbon which is further attached to three carbon atoms directly.

10.2 NOMENCLATURE OF ALKYL HALIDES

COMMON NAMES

Alkyl halides (monohaloalkanes) are named according to the nature of the alkyl group to which halogen atom is attached. For example,

CH 3 CH 2 OH + HX CH 3 CH 2 X + H 2 O

Ethyl halide

ZnCl 2

(b) Alcohols also react with thionyl chloride in pyridine as a solvent to give alkyl chlorides. This method is especially useful since the by-products (HC1, SO 2 ) are gases, which escape leaving behind the pure product.

ROH + SOCl 2 R Cl + SO 2 + H 2 O

Pyridine

1. From Alcohols (a) Reaction of alcohols with halogen acids. Alcohols may be converted to the corresponding alkyl halides by the action of halogen acid in the presence of ZnCl 2 which acts as a catalyst.

(c) Phosphorus trihalides or phosphorus pentahalides react with alcohols to replace -OH group by a halo group. 3CH 3 CH 2 OH + PBr 3 3CH 3 CH 2 Br + H 3 PO 3 CH 3 CH 2 OH + PCl 5 CH 3 CH 2 Cl + POCl 3 + HCl

2. An excellent method for the preparation of simple alkyl iodide is the treatment of alkyl chloride or alkyl bromide with sodium iodide. This method is particularly useful because alkyl iodides cannot be prepared by the direct iodination of alkanes. RCl Nal Rl + NaCl RBr Nal Rl + NaBr

10.4 REACTIVITY OF ALKYL HALIDES

An alkyl halide molecule (R — X) consists of two parts, an alkyl group with a partial positive charge on the carbon atom attached to halogen atom and the halide atom with a partial negative charge.

There are two main factors which govern the reactivity of R — X bond. These are: i) C— X Bond energy ii) C— X Bond polarity

Bond Energy The following table shows the bond energies of C— X bonds in alkyl halides. The strength of the bonds show that iodo compound (with the weakest bonds) would be the most reactive one while fluoro compound will be the least reactive i.e., the order of reactivity of alkyl halides should be

R— I > R— Br > R— Cl > R— F

Bond Polarity

Electronegativities of halogen, carbon and hydrogen atoms present in alkyl halides are shown in the table. The greatest electronegativity difference exists between carbon and fluorine atoms in alkyl fluorides.

Bond

Bond Energy (kj/mole) C — F C — H C — Cl C — Br C — I

467 413 346 290 228

Atom Electronegativity F Cl Br I H C

If an electrophile is the attacking reagent then this difference suggests that alkyl fluorides would be the most reactive one. On the same lines, alkyl iodides should be the least reactive alkyl halides. In the light of the above discussion it is clear that the two factors mentioned above predict different types of behaviour about the reactivity of alkyl halides.

So the overall order of reactivity of alkyl halides for a particular alkyl group is: Iodide > Bromide > Chloride > Fluoride In fact the C-F bond is so strong that alkyl fluorides do not react under ordinary conditions.

Experiments have shown that the strength of carbon halogen bond is the main factor which decides the reactivity of alkyl halides.

10.5 REACTIONS OF ALKYL HALIDES

The reactions of alkyl halides fall into two categories.

1. Those reactions in which the halogen is replaced by some other atom or a group (nucleophilic substitution, or SN reactions).
2. Tnose which involve the removal of HX from the nanae (elimination, or E reactions).

10.5.1 Nucleophilic Substitution Reactions

Before discussing specifically the nucleophilic substitution reactions (SN) of alkyl halides, let us look at the nucleophilic reaction in general. The overall process describing an SN reaction is shown as follows:

In this equation the incoming group Nu is a nucleophile. Nucleophile means nucleus loving. It has an unshared electron pair available for bonding and in most cases it is basic in character. It may be negatively charged or neutral.

Electrophile

Animation 10.3: Electrophilic addition Source & Credit: johnwiley

It is a specie which attracts electrons (electron loving). The carbon atom of an alkyl group attached with the halogen atom and bearing a partial positive charge is called an electrophile or electrophilic center. An electrophile may be neutral or positively charged.

Examples of Nucleophiles

HO- C 2 H 5 O- H S- SCN- H 2 O: NH 2 -

Hydroxide ion Ethoxide ion Hydrogen sulphide ion Thiocyanate ion Water Amino group

CI- Br - NH 3 CN- I-

Chloride ion Bromide ion Ammonia Cyanide ion Iodide ion

:

:

L is also a nucleopile. It is called leaving group because it departs with an unshared pair of electrons. If we wish a SN reaction to proceed in the forward direction the incoming nucleophile must be stronger than the departing one. Cl-^ , Br-^ , I-^ , HSO 4 -^ are good leaving groups. Poor leaving groups are OH-^ , and NH 2 -. Iodide ion is a good nucieophile as well as a good leaving group.

Leaving Group

Substrate Molecule The alkyl halide molecule on which a nucleophile attacks is called a substrate molecule.

OR

10.5.2 Mechanism of Nucelophilic Substuitution Reactions

Alkyl halides may undergo nucleophilic substitution reactions in two different ways:

1. Nucleophilic Substitution Bimolecular (SN2)
2. Nucleophilic Substitution Unimolecular (SN1)

In SN1 mechanism, the nucleophile attacks when the leaving group had already gone, so the question of the direction of the attack does not arise. Moreover, the intermediate carbocation is a planar specie allowing the nucleophile to attack on it from both the directions with equal ease. We, therefore, observe 50% inversion of configuration and 50% retention of configuration.

Reactions involving SN 1 mechanism show first order kinetics and the rates of such reactions depend only upon the concentration of the alkyl halide. The rate equation of such reactions can be written as follows. Rate = k [Alkyl halide] Tertiary alkyl halides when attacked by a nucleophile always follow S (^) N 1 mechanism. Secondary alkyl halides, on the other hand, follow both SN1 and SN 2 mechanisms.

10.5.3 b^ -Elimination Reactions

During nucleophilic substitution reactions, the attacking nucleophile attacks the electrophilic carbon atom of the alkyl halide. There is another site present in the alkyl halide molecule where the nucleophile can attack at the same time.Such a site is an electrophilic hydrogen atom attached to the^ b^ -carbon of the alkyl halide.

When the attack takes place on hydrogen, we get an alkene instead of a substitution product. Such a type of reactions are called elimination reactions.

These reactions take place simultaneously with substitution reactions and often compete with them.

Like nucleophilic substitutions, the elimination reactions can also follow E or E1 mechanism. In E2 mechanism, the nucleophile attacks and the leaving group leaves at the same time with a formation of carbon carbon double bond.

The single step E2 elimination

Like SN2 reactions, the molecularity of E2 reactions is also two and these reactions show second order kinetics. In E1 mechanism, like SN 1 mechanism, the first step is the slow ionization of the substrate to give a carbocation. In the second step, the nucleophile attacks on hydrogen to give an alkene as a product

E2 mechanism is a bimolecular mechanism and the rates of those reactions which follow this mechanism depend upon the concentrations of the alkyl halide as well as the attacking nucleophile or a base. E mechanism, on the other hand, is a unimolecular mechanism and the rates of those reactions which follow this mechanism depend only upon the concentration of the alkyl halide molecule. Primary alkyl halides generally follow E2 mechanism whereas tertiary alkyl halides follow E l mechanism. Examples of SN reactions are given below.These reactions show the usefulness of alkyl halides as synthetic reagents.

Substrate Attacking Nucleophile Product

δ^ + − δ CH 3 CH 2 Br + OH C 2 H 5 OH + Br Ethyl alcohol δ^ + − δ CH 3 CH 2 Br + I C 2 H 5 I + Br Ethy liodide δ^ + (^) δ− CH 3 CH 2 Br + CN C 2 H 5 CN + Br Propane nitrile δ^ + (^) δ− CH 3 CH 2 Br + NO 2 C 2 H 5 NO 2 + Br Nitroethane δ^ + (^) δ− CH 3 CH 2 Br + CH 3 O CH 3 CH 2 O CH 3 + Br Ethyl methyl ether δ^ + − δ CH 3 CH 2 Br + NH 3 C 2 H 5 NH 2 + HBr Ethylamine δ^ + − δ CH 3 CH 2 Br + CH 3 CH 2 NH 2 (CH 3 CH 2 ) 2 NH + HBr Diethylamine δ^ + (^) δ− CH 3 CH 2 Br + (CH 3 CH 2 ) 2 NH (CH 3 CH 2 ) 3 N + HBr Triethylamine δ^ + (^) δ− CH 3 CH 2 Br + (CH 3 CH 2 ) 3 N (CH 3 CH 2 ) 4 N + Br Quaternary ethylammonium ion δ^ + − δ CH 3 CH 2 Br + SH C 2 H 5 SH + Br Ethyl thioalcohol

δ^ + − δ CH 3 CH 2 Br + CH 3 COO Na +^ CH 3 COOC 2 H 5 + NaBr Ethyl acetate Other reactions shown by alkyl halides are as follows:

−

−

−

−

−

−

− − − − − −

1. Wurtz Synthesis

Alkyl halides react with sodium in ether solvent to give alkanes. The reaction is particularly useful for the preparation of symmetrical alkanes.

CH 3 CH 2 Cl + 2Na + Cl CH 2 CH 3 CH 3 CH 2 CH 2 CH 2 CH 3 + 2NaCl n-Butane

Ether

Alkyl halides can be reduced with zinc in the presence of an aqueous acid such as HCI or CH 3 COOH.

2. Reduction of Alkyl Halides

CH 3 CH 2 CH 2 Cl + Zn + H+^ + Cl CH 3 CH 2 CH 3 + ZnCl 2 Propane

4CH 3 Cl + Na 4 Pb (CH 3 ) 4 Pb + 4NaCl Tetramethyl lead 4CH 3 CH 2 Cl + Na 4 Pb (CH 3 CH 2 ) 4 Pb + 4NaCl Tetramethyl lead

Methyl chloride and ethyl chloride react with sodium lead alloy giving tetramethyl lead and tetraethyl lead,respectively. These compounds are important anti-knock agents and are used in gasoline.

3. Reaction with Sodium Lead Alloy (Na 4 Pb)

10.6 GRIGNARD REAGENT

Grignard reagents RMgX are derivatives of alkyl halides belonging to class of organo-metallic compounds. Grignard reagent was first prepared by Victor Grignard in 1900. These reagents are so important in organic synthesis that almost all the classes of organic compounds can be prepared from them. Due to their importance and applications Victor Grignard was awarded Nobel prize in chemistry.

(viii) With CH 3 COCH (^3)

(vii) With CH 3 CHO

(ix) With Epoxide

Simulation 10.3: Interactive Periodic Table Source & Credit: learnerstv

1. Monohalo derivates of alkanes are called alkyl halides.
2. The general formula of alky 1 halides is CnH2n +1X.
3. The best method for the preparation of alkyl halides is by the reactions of alcohols with inorganic halides like SOCl 2 , PX 3 and PX 5.
4. Alkyl halides are very reactive class of organic compounds. They undergo nucleophilic substitution reactions and elimination reactions in the presence of a nucleophile or a base.
5. Nucelophilic substitution reactions can take place in two distinct ways. A one step mechanism is called SN2 while a two step mechanism is called SN1. SN1 reactions show first order kinetics whereas SN2 reactions show 2 nd^ order kinetic.
6. Nucleophilic substitution reactions take place simultaneously with elimination reactions and often compete with them.
7. Elimination of two atoms or groups from adjacent carbon atoms in the presence of a nucleophile or a base is called elimination reaction. Like nucleophilic substitution, (3-elimination reaction also take place in two distinct ways E2 and E1.
8. Grignard reagent can be prepared by adding alkyl halide in a stirred suspension of magnesium metal in diethyl ether.
9. Grignard reagent has a reactive nucleophilic carbon atom which can react with electrophilic centres to give the products in high yields.
10. Primary, secondary and tertiary alcohols can be best prepared by reacting Grignard reagent with formaldehyde, any other aldehydes and ketones, respectively.

KEY POINTS

EXERCISE

ii) The best method for the preparation of alkyl halides is the reaction of _________ with inorganic reagents.

Q.1. Fill in the blanks. i) In tertiary alkyl halides the halogen atom is attached to a carbon which is further attached to __________ carbon atoms directly.

iii) An alkyl group with a partial positive charge on the carbon atom is called____________ centre. iv) The mechanism is called __________if it involves one molecule in the ratedetermining step. v) Molecularity of a reaction is defined as the number of molecules taking part in the ______________. vi) The molecularity of E2 reactions is always two and the reactions show ________ order kinetics. vii) Wurtz synthesis is useful for the preparation of____________ alkanes. viii) Grignard reagents are prepared by the reaction of magnesium metal with alkyl halides in the presence of___________.

Q.2. Indicate True or False.

i) In secondary alkyl halides, the halogen atom is attached to a carbon which is further attached to two carbon atoms directly. ii) Alcohols react with thionyl chloride in ether as solvent to give alkyl halides. iii) Order of reactivity of alkyl halides for a particular alkyl group is: Iodide > Bromide > Chloride > Fluoride iv) In SN2 reactions the attacking nucleophile always attacks from the side in which the leaving group is attached. v) Methyl magnesium iodide on hydrolysis yields ethyl alcohol. vi) Primary, secondary and tertiary amines react with Grignard reagents in the sameway. vii) The reactions of secondary alkyl halides may follow both SN1 and SN 2 mechanisms. viii) SN1 mechanism is a one stage process involving a simultaneous bond breakage and bond formation. ix) In^ b^ -elimination reactions, the two atoms or groups attached to two adjacent carbon atoms are lost under the influence of an electrophile. x) The reactivity order of alkyl halides is determined by the strength of carbon- halogen bond.

10.ALKYLHALIDES eLearn.Punjab

Q.7. Draw all the possible structures that have the molecular formula C 6 H 13 CI. Classify each as primary, secondary or tertiary chloride. Give their names according to IUPAC system.

Q.8. Using ethyl bromide as a starting material how would you prepare the following compounds. Give also the inorganic reagents and conditions necessary to carry out these reactions:

(a) n-Butane (b) Ethyl alcohol (c) Ethyl cyanide (d) Ethane (e) Ethene (f) Propanoic acid (g) Propane

Q.9. Write a detailed note on the mechanism of nucleophilic substitution reac- tions.

Q.10. What do you understand by the term b-elimination reaction. Explain brief- ly the two possible mechanisms of 3-elimination reactions.

Q.11. What products are formed when the following compounds are treated with ethyl magnesium bromide, followed by hydrolysis in the presence of an acid, i) HCHO ii) CH 3 CHO iii) CO (^2) iv) (CH 3 ) 2 CO v) CH 3 — CH 2 — CHO vi) CICN

Q. 12. How will you carry out the following conversions.

i) CH 4 CH 3 CH 2 COOH

ii) CH 3 — CH 3 (CH 3 — CH 2 ) 4 N+^ Br

iii) CH 2 = CH 2 CH 3 — CH 2 — CH 2 — CH 2 — OH

iv) CH 3 CH 2 CH 2 CI CH 3 — CH = CH 2

v) CH 3 COOH CH 3 CH 2 COOH