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Alkyl Halides notes semester 1, Study notes of Organic Chemistry

Imam Abdulrahman Bin Faisal University (IAU)Organic Chemistry

Alkyl Halides, Organic chemistry semester 1

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ORG. CHEM. I

(PHCH - 203)

ALKYL HALIDES

(R—X)

 INTRODUCTION

 STRUCTURE

 OCCURENCE

 NOMENCLATURE

 SYNTHESIS or METHODS OF PREPARARATION

 PROPERTIES (PHYSICAL & CHEMICAL)

 IMPORTANCE or APPLICATIONS

Discover Study notes of Organic Chemistry Imam Abdulrahman Bin Faisal University (IAU)

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ORG. CHEM. I

(PHCH - 203)

ALKYL HALIDES

(R—X)

 INTRODUCTION

 STRUCTURE

 OCCURENCE

 NOMENCLATURE

 SYNTHESIS or METHODS OF PREPARARATION

 PROPERTIES (PHYSICAL & CHEMICAL)

 IMPORTANCE or APPLICATIONS

INTRODUCTION and STRUCTURE

 The halo alkanes (also known as halogenoalkanes or alkyl halides ) are a group of

chemical compounds derived from alkanes containing one or more halogens.

 The General Formula:

R X

Here X = -F, -Cl, -Br, -I and R = Aliphatic or aromatic

Primary, Secondary and Tertiary carbons

H 3 C

CH 3

4 ^3

Hydrogen atoms attached to 1^0 , 2^0 and 3^0 carbon atoms are referred as primary, secondary

and tertiary hydrogen atoms.

Alkyl halides are classified as

Halomon isolated hornemannii^ from^ the^ red^ alga^ Portieria

and found to have anticancer activity

against several human tumor cell lines.

Herbicides

and many more ---------------

NOMENCLATURE

Alkyl halides are named by two systems

Formula Common System IUPAC System

CH 3 -Cl Methyl Chloride Chloromethane

CH 3 CH 2 - Cl Ethyl chloride Chloroethane

CH 3 CH 2 CH 2 - Cl n-propyl chloride Chloropropane

COMMON SYSTEM

 In this system alkyl group attached to the halogen atom is/are named first, and followed by

word chloride, bromide, fluoride and iodide.

Examples-

H 3 C Cl H 3 C CH 2 Cl H 3 C CH 2 CH 2 Cl H 3 C CH Cl

CH 3

H 3 C CH 2 CH 2 CH 2 Cl H 3 C CH 2 HC Cl

CH 3

C Cl

H 3 C

Methyl chloride (^) Ethyl chloride Propyl chloride (^) iso- propyl chloride

Butyl chloride (^) Iso-butyl chloride neo-butyl chloride sec-butyl chloride tert-butyl chloride

IUPAC (International Union of Pure and Applied Chemistry) System

In the IUPAC system alkyl halides are named as HALOALKANES.

Rule 1. Select the longest continuous carbon chain to which halogen group is attached.

Rule 2. Number the chain-

The numbering is started from that carbon which having halogen atom on the lowest number.

CH 3 -CH 2 CH 2 -CH 2 - Cl

1-Chlorobutane

Rule 3. Number the substituents according to their positon on the carbon chain Number

the chain so as to give the carbon having halogen atom at the lowest possible number.

CH2 CH 2 C CH 3

CH 3

H 3 C

2-Chloro-2-methyl pentane

H 3 C CH 2 CH 2 Cl

ClH 2 C CH 2 CH 2 Cl

H 3 C CH CH 3

Cl (^321)

1-Chloropropane (^) 1,3-Dichloropropane

2-Chloropropane

Rule 4. Number the carbons of the parent chain beginning at the end nearer the first

substituent, whether alkyl or halo.

 Assign each substituent a number according to its position on the chain.

ISOMERISM – they undergoes positional isomerism.

Positional Isomerism –

Have the same molecular formula but differ in the position of functional group arrangements

of the carbon chain.

Example

CH 3 CH 2 CH 2 Cl CH 3 CH (Cl) CH 3

1-Chroropropane 2-Chloropropane

CH 3 CH 2 CH 2 CH 2 Cl CH 3 CH (Cl) CH 2 CH 3

1-Chrorobutane 2-Chroropropane

METHODS OF PREPARATIONS or SYNTHESIS

1. Halogenation of alkanes –

 It is a type of FREE-RADICAL halogenation

 It proceeds by a free-radical mechanism.

FREE RADICALS

They are formed by homolytic fission or homolysis of covalent bonds.

Cl Cl Cl.^ + Cl.

Here both chlorine atoms have similar electro negativities.

H

Z H

H

. (^) +.^ Z

Here Z and carbon atom have similar electro negativities.

Example

H

. H 3 C

H

Methyl free radical Ethyl free radical

Chlorination:

 Alkanes react with Cl 2 in the presence of sunlight or UV light or at high temperature

(300-400 0 C).

CH 4 + Cl 2

UV light CH

3 Cl^ + HCl

Excess major product

FREE RADICAL MECHANISM

Completed into 3 steps

1. Initiation

2. Propagation

3. Termination

1. Initiation step: - Formation of free radicals.

A chlorine molecule undergoes homolytic fission in the presence of UV- light to give

chlorine free radicals.

2. Propagation step: - A chlorine free radicals are attracting with methane to form a

methyl free radical and HCl.

The methyl free radical attacks a chlorine molecule to give methyl chloride and

chlorine free radical.

3. Termination step:- The above steps stops when any two free radicals combine to

each other.

The overall reaction is:

5. Reaction of Phosphorous Halides with alcohol

Alcohols react with phosphorous trihalides (PX 3 ) and pentahalides (PX 5 ) to form

alkyl halides.

R OH (^) + PX 3 R^ X + H 3 PO 3 Alkanol (^) Alkyl halide

H 5 C 2 OH (^) + PCl 3 H 5 C 2 Cl + H 3 PO 3 Ethanol (^) Ethyl Chloride

R OH (^) + PX 5 R^ X + POX 3 Alkanol (^) Alkyl halide

H 5 C 2 OH (^) + PCl 5 H 5 C 2 Cl + POCl 3 Ethanol (^) Ethyl Chloride

6. Reaction of Thionyl Chloride with alcohol

Alcohols react with thionyl chloride (SOCl 2 ) to form alkyl halides.

R OH (^) + SOCl 2 R^ Cl + SO 2

Alkanol Alkyl chloride

H 5 C 2 OH (^) + SOCl 2 H 5 C 2 Cl + SO 2

Ethanol Ethyl Chloride

PROPERTIES

Physical properties

 Lower alkyl halides having C 1 - C 2 ----------- gases

 Other alkyl halide up to C 18 ---------- colorless liquid

 Above C 18 ------------ colorless solids

Solubility-

 are insoluble in water (because unable to form hydrogen bond with water molecule)

but soluble in organic solvents.

Boiling point –

 Boiling point increases with increasing molecular weight of the compound.

Chemical Properties

 Alkyl halides are very reactive compound. They undergo

SUBSTITUTION, ELIMINATION and REDUCTION reaction

 They also react with metals to form organometallic compounds

A. SUBSTITUTION REACTIONS

 The Carbon-halogen bond in alkyl halides is polar because of high

electronegativity of the halogen atom as compared to carbon.

 So that the carbon atom is therefore a good target for attack by nucleophiles (Nu-).

 In fact, the nucleophilic substitution reactions are the most common reactions of

alkyl halides.

The above reaction completed by two ways

SN1 and SN 2

SN1 Reaction (unimolecular nucleophilic substitution)

Mechanism – completed into two steps

H 3 C CH 3

H 3 C OH

H^ Br^ H 3 C CH 3

H 3 C Br

H 2 O

The attack of the nucleophile and the ejection of the halide ion take place simultaneously

Note –

In case of SN2 mechanism reaction rate follow in this order

Leaving group in SN 2 reaction

1. Reaction with Aq. KOH or NaOH–

R X

KOH

heat R^ OH^ +^ KX Alkyl halide Alkanol

H 3 C Cl

KOH

heat H^3 C^ OH^ +^ KCl Methyl chloride Methanol

R X (^) + NaOH R OH + NaX

NaOH +^ NaBr

Alkyl halide (^) Alkanol

Ethyl bromide Ethyl alcohol

X = -Cl, -Br, -I

H 2 O

heat

CH 3 CH 2 Br (^) heat CH 3 CH 2 OH

2. Reaction with Aq. KSH–

R X

KSH

heat R^ SH^ +^ KX Alkyl halide Alkanethiol

H 3 C Cl

KSH

heat H^3 C^ SH^ +^ KCl Methyl chloride methanethiol

3. Reaction with moist silver oxide (AgOH)–

Ag 2 O

H 2 O

2AgOH

R X

AgOH heat R^ OH^ +^ AgX Alkyl halide Alkanol

H 3 C Br

AgOH heat H^3 C^ OH^ +^ AgBr Methyl bromide Methanol

4. Reaction with dry silver oxide (Ag 2 O)–

R X

Ag 2 O heat R^ O^

R

2AgX Alkyl halide Ether

H 3 C I

Ag 2 O heat H^3 C^ O

CH 3

2AgI Methyl iodide Diethyl ether

5. Reaction with sodium alkoxide (RONa)–

ROH

Na RONa

R X

RONa heat R^ O^

R

NaX Alkyl halide Ether

H 3 C Br

RONa heat H^3 C^ O

CH 3

NaBr Methyl bromide Diethyl ether

Note - This synthesis is called as Williamson ether synthesis

6. Reaction with pot. sulphide (K 2 S)–

R X

K 2 S

heat R^ S^

R

2KX Alkyl halide Thioether

I

H 3 C K 2 S

heat H^3 C^ S

CH 3

2KI Ethyl iodide Diethyl sulphide

B. ELIMINATION REACTIONS

1. Reaction with alcoholic KOH–

KOH/ethanol heat H^2 C^ EthyleneCH^2 +^ KBr

H 3 C

Ethyl bromide

H^2 O

KOH/ethanol heat CH^2

H 3 C

KBr Propene

H 3 C

1-bromopropane

H^2 O

H 3 C CH 2 HC CH 3

Br KOH/Et-OH

H 3 C CH 2 CH CH 3

H 3 C CH 2 CH CH 2

1-butene (minor)

2-butene (major)

2-bromobutane

Note- Zaitsev's Rule - Highly substituted Alkene is more stable than other

C. OTHER REACTIONS

1. Reduction with reducing agents like -----

Zn+ HCl, LiAlH 4 /ether and H 2 /Ni/Pd -------- give alkane

H 3 C (^) Zn/HCl H 3 C CH 3 + HCl Ethyl chlolride Ethane

H 3 C (^) LiAlH 4 H 3 C CH 3 + HCl Ethyl chlolride Ethane

H 3 C (^) Ni or Pd H 3 C CH 3 + HCl Ethyl chlolride Ethane

ether

2. Reaction with Mg in dry ether – to form Grignard reagents

Note -

Grignard reagents react with aldehydes or ketones to form an addition compound

which on hydrolysis with dilute acid gives the corresponding alcohols.

O (^) + R MgX R OMgX R^ OH + MgX(OH) Aldehyde or Ketone

G.R

H 2 O / H+

Unstable^ Alcohol

-^ -^ +

Formaldehyde gives primary alcohol

Acetaldehyde gives secondary alcohol

Ketone gives tertiary alcohol

Formaldehyde gives primary alcohol

O

H

R^ MgX^ OMgX

H

R OH

H

R (^) + MgX(OH)

Formaldehyde

G.R

H 2 O / H+

Unstable^10 Alcohol

-^ -^ + ether

Acetaldehyde gives secondary alcohol

H 3 C

R^ MgX^ OMgX

H 3 C

R OH

H 3 C

R (^) + MgX(OH)

Acetaldehyde

G.R

H 2 O / H+

Unstable^20 Alcohol

-^ -^ + ether

Ketone gives tertiary alcohol

O

H 3 C

R^ MgX^ OMgX

H 3 C

R OH

H 3 C

R (^) + MgX(OH)

Acetone or ketone

G.R

H 2 O / H+

Unstable^30 Alcohol

-^ -^ + ether

5. Friedel–Crafts Alkylation Reaction

 The introduction of an alkyl group onto the benzene ring.

 The reaction is carried out by treating the aromatic compound with an alkyl halide

(RX), in the presence of AlCl 3 to generate a carbocation or electrophile (R+).

 Aluminum chloride (AlCl 3 ) catalyzes the reaction it helps to dissociate the alkyl

halide.

REACTION

AlCl 3

CH 3

H^ Cl Toluene

H 3 C Cl

Benzene

MECHANISM

The above reaction completed into 3 steps

STEP I- Formation of an Electrophile

H 3 C Cl

AlCl (^3) CH 3

Electrophile

AlCl 4 -

STEP II- Attack of an Electrophile on aromatic ring to form CARBONIUM ION

CH^3

Electrophile

CH 3

+^ H

Benzene Carbonium ion

STEP III- Loss of proton from carbonium ion to give the substituted product.

CH 3

+^ H

Carbonium ion

CH 3

H^ Cl^ +

AlCl 4 - AlCl 3

Toluene

Alkyl Halides notes semester 1, Study notes of Organic Chemistry

Related documents

Partial preview of the text

Download Alkyl Halides notes semester 1 and more Study notes Organic Chemistry in PDF only on Docsity!

ORG. CHEM. I

(PHCH - 203)

ALKYL HALIDES

(R—X)

 INTRODUCTION

 STRUCTURE

 OCCURENCE

 NOMENCLATURE

 SYNTHESIS or METHODS OF PREPARARATION

 PROPERTIES (PHYSICAL & CHEMICAL)

 IMPORTANCE or APPLICATIONS

INTRODUCTION and STRUCTURE

 The halo alkanes (also known as halogenoalkanes or alkyl halides ) are a group of

chemical compounds derived from alkanes containing one or more halogens.

 The General Formula:

R X

Primary, Secondary and Tertiary carbons

H 3 C

H 3 C

H 3 C

CH 3

CH 3

4 ^3

Hydrogen atoms attached to 1^0 , 2^0 and 3^0 carbon atoms are referred as primary, secondary

and tertiary hydrogen atoms.

Alkyl halides are classified as

Halomon isolated hornemannii^ from^ the^ red^ alga^ Portieria

and found to have anticancer activity

against several human tumor cell lines.

Herbicides

NOMENCLATURE

Alkyl halides are named by two systems

Formula Common System IUPAC System

CH 3 -Cl Methyl Chloride Chloromethane

CH 3 CH 2 - Cl Ethyl chloride Chloroethane

CH 3 CH 2 CH 2 - Cl n-propyl chloride Chloropropane

COMMON SYSTEM

 In this system alkyl group attached to the halogen atom is/are named first, and followed by

word chloride, bromide, fluoride and iodide.

Examples-

CH 3

CH 3

CH 3

H 3 C

H 3 C

IUPAC (International Union of Pure and Applied Chemistry) System

In the IUPAC system alkyl halides are named as HALOALKANES.

Rule 1. Select the longest continuous carbon chain to which halogen group is attached.

Rule 2. Number the chain-

The numbering is started from that carbon which having halogen atom on the lowest number.

CH 3 -CH 2 CH 2 -CH 2 - Cl

1-Chlorobutane

Rule 3. Number the substituents according to their positon on the carbon chain Number

the chain so as to give the carbon having halogen atom at the lowest possible number.

CH2 CH 2 C CH 3

CH 3

H 3 C

H 3 C CH CH 3

Rule 4. Number the carbons of the parent chain beginning at the end nearer the first

substituent, whether alkyl or halo.

 Assign each substituent a number according to its position on the chain.

ISOMERISM – they undergoes positional isomerism.

Positional Isomerism –

Have the same molecular formula but differ in the position of functional group arrangements

of the carbon chain.

Example

CH 3 CH 2 CH 2 Cl CH 3 CH (Cl) CH 3

1-Chroropropane 2-Chloropropane

CH 3 CH 2 CH 2 CH 2 Cl CH 3 CH (Cl) CH 2 CH 3

1-Chrorobutane 2-Chroropropane

METHODS OF PREPARATIONS or SYNTHESIS

1. Halogenation of alkanes –

 It is a type of FREE-RADICAL halogenation

 It proceeds by a free-radical mechanism.

FREE RADICALS

They are formed by homolytic fission or homolysis of covalent bonds.