General Properties
Group 17 (VII) Elements:
Halogens
Electronegative, Reactive and
Here to Stay…
Stay…
• The most electronegative group - withdrawing
ability (enhances Bronsted acidity),
electronegativity decreases down the group
• Boiling points and melting points are relatively
low and increase down the group
F
Cl
Br
I
At
Electronegativity
4.0
3.0
2.8
2.5
2.2
Melting point / deg C
-220
-101
-7.2
114
302
Boiling point/deg C
-188
-34.7
58.8
184
• Colors include nearly colorlesscolorless-yellow (F),
yellow), redyellow-green (Cl
(Cl),
red-brown (Br), and purple
(I)
• “That’
That’s color. That’
That’s fun.”
fun.” ~Bob
States of Halogens
At room
temperature,
fluorine is in
gaseous form
At room
temperature,
Bromine is in
liquid form .
At room
temperature,
chlorine is in
gaseous form
At room
temperature,
Iodine is in
solid form
Intermolecular forces are primarily responsible
for the differences in the states of the elements.
The larger more polarizable atoms induce dipole
moments more easily and are less volatile.
More General Properties
• Covalent and ionic radius: slightly lower than
Group 16, decreases down group
• First ionization energy, highest except for Noble
gases, decreases down the group
• Electron affinity, highest vs. other groups, Cl has
highest Ea, but generally decreases down
group.
F
Cl
Br
I
At
Covalent radius/pm
72
99
114
133
Ionic radius /pm
117
167
182
206
First ionization energy
(kJ/mol--1)
15680
1250
1140
1008
Electron affinity (kJ/mol-1)
334
355
325
295
140
270
1
Fluorine is Special
General Reactivity
• Most reactive nonmetal group
• Bond dissociation
enthalpy trends
• Oxidation numbers
are typically -1, can
have +1, +3, +5, &
+7 oxidation states
• Note: F exception
Bond Dissociation Enthalpy (Shriver & Atkins Fig. 16.5)
300
Bo n d Dis. enth alp y (kJ/m o l)
250
243
193
200
159
151
150
100
50
0
F2
Cl2
Br2
Halogens
Hydrogen Fluoride Fun
•
•
•
•
Less acidic than other hydrogen halides
Volatile liquid vs. gas (Higher BP)
High electrical conductivity
Hydrogen Bonding: Linear (No 3D
Network which exists in H20)
• Excellent nonnon-aqueous solvent
• Very Dangerous – Readily absorbs
through skin: toxic and corrosive
I2
• The most electronegative element…
element…
ever…
ever…
• The strongest oxidizing agent besides
Oxygen. Raises cations to high oxidation
states.
• Fluorine is distinctive: strong Coulombic
repulsion, explains lower electron affinity
and bond dissociation
• Never has positive oxidation state
Halogen Gas Formation
• All halogen diatomic gases are produced
differently…
differently…
• Bromine gas comes from chemical
oxidation of Br - ions in seawater
• Fluorine gas isolated from electrolysis of
1:2 molten KF and HF
2
Industrial Applications
• Fluorine gas stored in steel or monel metal
(Ni/Cu alloy) which forms passivating
metal fluoride surface film
• Dr. Leland Clark – Perfluorocarbons
CH3
H3C
C
CF 3
CH3
O
H2
C
F2/He
CH
F 3C
C
tert-butyl isobutyl ether
CH3
• Grignard Reagents - Mg & Halogens
• A powerful synthetic tool to create a variety of
polyfunctional heterocycles
• Sensitive groups such as nitro, hydroxy and
boronic ester.
CF3
O
F2
C
OMe
CF
OMe
N
CF 3
Ph
N
Industrial Applications
• Using Halogens in the Combustion of
Coal
• Halogens react with oxygen free
radicals in fluidized bed combustion
(FBC).
• Greatly increases the CO and
hydrocarbon concentrations in the
gases when salt halides are added to
FBC. - CO Æ CO2 does not occur
H
Ph
N
Br
OMe
• Oxycyte - High Oxygen Carrying Capacity,
Possible Use as Artificial Blood
N
+
CF3
perfluoro(tert-butyl isobutyl ether)
OH
O
MgCl
-120 C
CH3
Industrial Applications
Br
OMe
• Professor Ila - Preparation and Reactions of Heteroaryl
Organomagnesium Compounds
Industrial Applications
• Halogens increase NO concentrations
because the reaction NO Æ N2O
involves free radicals.
• I2 is better at decreasing the oxygen
free radicals than Cl2
3
Environmental Applications
• Volatile organohalogens break down
ozone.
• Some volatile organohalogens;
organohalogens;
including methyl chloride, methyl
bromide, methyl iodide, chloroform,
and bromoform;
bromoform; form highly reactive
halogen radicals.
Environmental Applications
• These volatile organohalogens are
produced by marine macroalgae.
macroalgae.
• It has been discovered that UV light
actually catalyzes the production of
volatile organohalogens by
macroalgae.
macroalgae.
• The problem is cyclical.
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