chapter 8 alkyl halides
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8. Alkyl Halides
Dr Wong Yau Hsiung CHEM 221
McMurry Organic Chemistry 6th edition, Chapter 10 (c) 2003
2
What Is an Alkyl Halide/ Haloalkanes An organic compound containing at least one carbon-
halogen bond (C-X) X (F, Cl, Br, I) replaces H
Can contain many C-X bonds Properties and some uses
Fire-resistant solvents Refrigerants Pharmaceuticals and precursors
• Halogenoalkanes are similar to alkanes but with one or more of the hydrogen atoms replaced by a halogen.
• Halogenoalkanes can contain more than one type of halogen. For example, CFCs (chlorofluorocarbons) contain both chlorine and fluorine atoms. chloro-pentafluoroethane
trichloromethane
What are Haloalkanes?
• Name is based on longest carbon chain (contains double or triple bond if present)
• Number the carbons of the parent chain beginning at the end nearer the first substituent, whether alkyl or halo
• If more than one of the same kind of halogen is present, use prefix di, tri, tetra
• If there are several different halogens, number them and list them in alphabetical order
Naming Haloalkane
• Naming if two halides or alkyl are equally distant from ends of chain
- Begin at the end nearer the substituent whose name comes first in the alphabet
Naming Haloalkane
A chain of carbon atoms can be represented by R when drawing the structure. This is referred to as an R group.
Primary (1°) halogenoalkanes have one R group attached to the carbon linked to the halogen.
Secondary (2°) halogenoalkanes have two R groups attached to the carbon linked to the halogen.
Tertiary (3°) halogenoalkanes have three R groups attached to the carbon linked to the halogen.
Primary, secondary and tertiary
R-CH2-X
R2-CH-X
R3-C-X
Physical Properties Solubility : All organic halides are insoluble in water and soluble in common organic solvents.Boiling point : The boiling points increases with increasing in
molecular weights. Therefore, the boiling points increases in the order F<Cl<Br<I.
BP also increases for “straight” chain isomers. Greater branching = lower BP
How are halogenoalkanes made?
There are several ways by which halogenoalkanes can be made, including: free radical substitution of an alkane:
electrophilic addition of HX or X2 to an alkene:
CH4 + Cl2 CH3Cl + HCl
C2H4 + HBr C2H5Br
C2H4 + Br2 C2H4Br2
How are halogenoalkanes made?
• The carbon–halogen bond in halogenoalkanes is polar because all halogens are more electronegative than carbon.
• The polar bond means that the carbon atom has a small positive charge (δ+), which attracts substances with a lone pair of electrons. These are nucleophiles, meaning ‘nucleus (positive charge) loving’. Examples include:
δ+ δ- δ+ δ- δ+ δ- δ+ δ-
ammonia cyanide hydroxide
Polar bonds and nucleophiles
• The nucleophile uses its lone pair to provide the electrons for a new bond
• The halogen is displaced • The result is substitution following attack by a nucleophile
Nucleophilic Substitution
• The general form for the reaction is Nu:- + R-X R-Nu + X:
Nucleophile Substrate Product Leaving group
Reagent Aqueous sodium (or potassium) hydroxide
Conditions Reflux in aqueous solution (SOLVENT IS IMPORTANT) Elimination takes place when ethanol is the solvent
The reaction with water is known as HYDROLYSIS
Product Alcohol
Nucleophile hydroxide ion (OH¯)
Equation C2H5Br(l) + NaOH(aq) —> C2H5OH(l) + NaBr(aq)
Nucleophilic Substitution – Aqueous NaOH
Nucleophilic Substitution Bimolecular or SN2• The rate depends on the conc. of 2 reactants: the substrate and
the nucleophile. • Takes place in one step • No carbocation • new bond forming and old bond breaking at same time.• Occur most readily with methyl compounds and primary
haloalkanes
H3CH2C
C
CH3H
BrOH-
H3CH2C
C
CH3H
BrOH
CH2CH3
C
CH3H
OH
Transition State: As OH- attaches, Br- leaves
+ Br-+
Nucleophilic Substitution Bimolecular or SN2• Transition state is highest in energy.
Nucleophilic Substitution Unimolecular or SN1• The rate depends on the conc. of 1 reactant: the substrate but not
the nucleophile.• A two step process since the substrate and the nucleophile cannot
both appear in the rate determining step• Form carbocation intermediate• Occur most readily with tertiary haloalkanes and some secondary
haloalkanes.
H3CH2C
C
CH3H
Br
H3CH2C
C+
CH3H
CH2CH3
C
CH3H
OH
Transition State: Formation of Carbocation
Br-OH-
+
Rate determining step: spotaneous dissociation of leaving group
Very fast step: reaction of nucelophile and carbocation
Nucleophilic Substitution Unimolecular or SN1
SN1 and SN2
SN1 SN2
Rate =k[RX] =k[RX][Nuc:-]
Carbocation intermediate?
Yes No
Number of steps 2 1
Occurs with Tertiary halogenoalkanes
Primary halogenoalkanes
• In the reaction with a strong base, halogenoalkanes will undergo not only nucleophilic substitution but also elimination reactions, forming alkenes and water.
• The OH- acts as both a base and a nucleophile. When acting as a base, the OH- removes H+ from the halogenoalkane, which also results in the formation of a halide ion.
• The reaction between a halogenoalkane and a strong base usually results in the formation of a mixture of substitution and elimination products.
Elimination in Haloalkanes
Reagent Alcoholic sodium (or potassium) hydroxide
Conditions Reflux in alcoholic solution
Product Alkene
Mechanism Elimination
Equation C3H7Br + NaOH(alc) —> C3H6 + H2O + NaBr
Complication With unsymmetrical halogenoalkanes, you can get mixture of products
Elimination
• If the carbon chain is four or more carbons in length and the halogen is not attached to a terminal carbon, a mixture of positional isomers may be formed.
attack at Aattack at BA B
but-2-ene but-1-ene
Mixture of elimination products
can exist as cis and trans isomers
Zaitsev’s Rule for Elimination Reactions • In the elimination of HX from an alkyl halide, the more highly
substituted alkene product predominates • If more than one elimination product is possible, the most-
substituted alkene is the major product (most stable).• R2C=CR2 > R2C=CHR > RHC=CHR > H2C=CHR
tetra > tri > di > mono
The products of reactions between haloalkanes and OH¯ are influenced by the solvent
SOLVENT ROLE OF OH– MECHANISM PRODUCT
WATER NUCLEOPHILE SUBSTITUTION ALCOHOL
ALCOHOL BASE ELIMINATION ALKENE
Modes of attack
Aqueous solution •OH¯ attacks the slightly positive carbon bonded to the halogen.•OH¯ acts as a nucleophile Alcoholic solution •OH¯ attacks one of the hydrogen atoms on a carbon atom adjacent the carbon bonded to the halogen.•OH¯ acts as a base (PROTON ACCEPTOR)
Both reactions take place at the same time but by varying the solvent you can influence which mechanism dominates.
Elimination vs Substitution
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