[PDF] Organic Chemistry II 9 sept. 2019 Classification of

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Prof. Mohamed El-Newehy

Chemistry Department, College of Science, King Saud University

CHEM 241

Organic Chemistry II

FOR CHEMISTRY' STUDENTS, COLLEGE OF SCIENCE

PRE-REQUISITES COURSE; CHEM 240

CREDIT HOURS; 2 (2+0)1

Organo-halogen CompoundsBased on NCERTOn the basis of number of halogen atoms; Haloalkanesandhaloarenesmay be classified as follows:

Classification of Haloalkanes and Haloarenesmono,di,orpolyhalogen(tri-,tetra-, etc.) compoundsdepending on whether they contain one, two or more halogen atoms in

their structures.

Monohalocompoundsmay further be classified according to the hybridization of the carbon atom to which the halogen is

bonded, as following:The replacement of hydrogen atom(s) in a hydrocarbon, aliphatic or aromatic, by halogen atom(s) (X=F,Cl,Br,orI;

fluoro, chloro, bromo, and iodo compounds) results in the formation of alkyl halide (haloalkane) and aryl halide

(haloarene) (a)Alkyl halides or haloalkanes(R - X) oThe halogen atom is bonded to an alkyl group (R) & thegeneral formulaisC n H 2n+1 X. (b)Allylic halides oThe halogen atom is bonded to ansp 3 -hybridized carbon atom next to carbon-carbon double bond (C=C).

Compounds Containing sp

3

C - X Bond (X= F, Cl, Br, I)

Classification of Haloalkanes and Haloarenes

(c)Benzylic halides oThe halogen atom is bonded to ansp 3 -hybridized carbon atom next to an aromatic ring.

oThey are classified asprimary,secondaryortertiaryaccording to the nature of carbon to which halogen is attached.

(a)Vinylic halides oThese are the compounds in which the halogen atom is bonded to ansp 2 -hybridized carbon atom of a carbon-carbon double bond (C = C). (b)Aryl halides oThese are the compounds in which the halogen atom is bonded to thesp 2 -hybridized carbon atom of an aromatic ring.

Compounds Containing sp

2

C - X Bond (X= F, Cl, Br, I)

p &O

Classification of Haloalkanes and Haloarenes

oCommon names;alkyl halides are derived by naming the alkyl group followed by the halide.Alkyl halide

oIUPAC system;alkyl halides are named ashalosubstituted hydrocarbons.

Haloalkanes

Nomenclature of Haloalkanes and Haloarenes

Haloarenes

oHaloarenes are the common as well as IUPAC names ofaryl halides.

oFor dihalogen derivatives, the prefixeso-,m-,p- are used in common system but in IUPAC system, the numerals

1,2; 1,3 and 1,4 are used.

%U &O+

Nature of C-X Bond C-X Bond

oSince halogen atoms are more electronegative than carbon, thecarbon-halogen bond of alkyl halide is polarized;

thecarbon atom bears a partial positive chargewhereas thehalogen atom bears a partial negative charge.

Dipole moment

(kJ/mol)Bond Length (Å)Bond

1.8474521.39CH

3 -F

1.8603511.78CH

3 -Cl

1.8302931.93CH

3 -Br

1.6362342.14CH

3 -ICarbon-Halogen (C - X) Bond Lengths, Bond Strength and Dipole Moments

Importance of Organohalogen Compounds

a) Solvents:Alkyl halides are used as solventsfor relatively non-polar compounds: CCl 4 , CHCl 3 , CCl 3 CH 3 , CH 2 Cl 2 , ....etc. b) Reagents: Alkyl halides are used as the starting materialsfor the synthesis of many compounds.

Alkyl halides are used in nucleophilic reactions, elimination reactions, formation of organometallics, and etc.

c) Refrigerants:Freons (ChloroFluoroCarbon) d) Pesticides:DDT, Aldrin, Chlordan

e) Herbicides: Kills broad leaf weeds but allow narrow leaf plants to grow unharmed and in greater yield

2,4-D

2,4-dichlorophenoxyacetic acidThey are good solvents for fats and oils.

Polychloro compounds, such as trichloroethylene and tetrachloroethylene, are widely used as solvents for dry cleaning.

f) Chlorine containing antibiotic, chloramphenicol: is very effective for the treatment of typhoid fever.

g) Our body produces iodine containing hormone, thyroxine: the deficiency of which causes a disease called goiter.

IodideBromideChlorideFlourideGroup

Density (g mL

-1 )bp (°C)Density (g mL -1 )bp (°C)Density (g mL -1 )bp (°C)Density (g mL -1 )bp (°C) 2.28 20

42.51.73

0

3.60.92

20 -23.80.84 -60 -78.4Methyl 1.95 20

721.46

20

38.40.91

15

13.10.72

20 -37.7Ethyl 1.74 20

1021.35

20

70.80.89

20

46.60.78

-3 -2.5Propyl 1.61 20

1301.27

20

1010.89

20

78.40.78

20

32Butyl

2.04 20

561.52

14

160.91

20 -13.90.68 26
-72CH 2 =CH- 1.84 22

102-1031.40

20

700.94

20

45-3CH

2 =CHCH 2 1.82 20

1891.52

20

1551.10

20

1321.02

20 85C
6 H 5 1.73 25
93
10 1.44 22

2011.10

25

1791.02

25
140C
6 H 5 CH 2 The simplemonofluoroandmonochlorocompounds areless dense than water, Themonobromoandmonoiododerivatives have densitiesgreater than water. As the number of halogen atoms increases, the density increases.

Density

All organic halidesare insoluble in water.

All organic halidesare soluble in common organic solvents (benzene, ether, etc.).

Solubility

Physical Properties of Organic Halides

Boiling points

Within a series of halides, the boiling points increase with increasing molecular weights. This is due to the increase in van der Waals forces when the size and mass of the halogen atom increases. Within a homologous series, the boiling points also increase regularly with molecular weights. Withinaseriesofisomers, the straight-chain compound has the highest boiling point, and the most branched isomer the lowest boiling point. Haloalkanes generally have a boiling point that is higher than the alkane they are derived from due to - The increased molecular weight due to the large halogen atoms - The increased intermolecular forces due to the polar bonds, and the increasing polarizabilty of the halogen.

Physical Properties of Organic Halides

Preparation of Halogen Compounds

Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes,

From Hydrocarbons

(a)By free radical halogenation;Alkyl halide oRate of halogenation:

Preparation of Halogen Compounds

(i)Addition of hydrogen halides:an alkene is converted to correspondingalkyl halideby reaction with hydrogen chloride,

hydrogen bromide or hydrogen iodide.

From Hydrocarbons

(b)From alkenes (ii) Addition of halogens: addition of bromine in CCl 4quotesdbs_dbs21.pdfusesText_27

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