Determination Of Boiling Point Of Organic Compounds ra5m

Dotsha J. Raheem Department of Chemistry College of Science University of Salahaddin

Definition of b.p. The boiling point of a liquid is the temperature at which its vapor pressure is equal to the pressure of the gas above it

 In of intermolecular interactions the b.p.

represents the energy required to overcome the various intermolecular attractions binding the molecules as a liquid (e.g. dipole-dipole attraction and hydrogen bonding) and therefore undergo a phase change into the gaseous phase . Therefore the boiling point of a liquid is also an indicator of the strength of the attractive forces between the liquid’s molecules.

 The normal boiling point of a liquid is the temperature

at which its vapor pressure is equal to 1 atm

Importance of b.p. B.p. is a physical constant used for: 1. Identification of unknown liquid compound 2. Purification

factors affecting b.p.  Pressure  Molecular weight  Structure of the molecule & intermolecular

interactions  Branching  Polarity  Van der Waals interactions  H-bonding  Impurities

Factors affecting B.P.

a. When the pressure is less than 1 atm , the boiling point of the liquid is less than its normal b.p. b. P = 1 atm, the b.p. of the liquid is called normal boiling point c. When P is greater than 1 atm, the b.p. of the liquid is greater than its normal b.p.

The higher the altitude, the lower the temp at which water boils

Factors affecting B.P.

CH4 and CH3Cl

H

H

H

C

C

H H

H

Cl H

Formula

IUPAC Name

Boiling Point

HCO2H

methanoic acid

101 oC

CH3CO2H

ethanoic acid

118 oC

CH3CH2CO2H

propanoic acid

141 oC

CH3(CH2)2CO2H

butanoic acid

164 oC

CH3(CH2)3CO2H

pentanoic acid

186 oC

CH3(CH2)4CO2H

hexanoic acid

205 oC

CH3(CH2)5CO2H

heptanoic acid

223 oC

Factors affecting B.P.

Within the branched series, increased symmetry leads to higher melting point and lower boiling point

Factors affecting B.P.

 CH4 (b.p. -161.5) , CH3-O-CH3 (b.p. -24)  Butane (-0.6 oC) , acetone (56 oC)

Factors affecting B.P.

 pentane (36), isopentane (31) and neopentane (9)

Pentane bp 36 oC

2,2-dimethylpropane bp 9.5 oC

OH

1-butanol (118),

n-butanol

OH

2-butanol (100)

isobutanol

OH

2-mehtyl-2-propanol (83)

tert-butanol

Factors affecting B.P.

 HF higher than HCl by 100 oC  H2O higher than H2S by 160 oC

H2O ( M.wt.=18): boiling point 100 oC H2S ( M.wt.=34): boiling point -60 oC  CH3OH (66), CH3SH (6)  CH3-O-CH3 (-24), CH3-S-CH3 (38)

b.p. is supposed to increase directly with m.wt., however, this is not the case for H2O, NH3 and HF

Compound

Structure

Boiling point

Acetamide

CH3C(O)NH2

222 oC

Acetic acid

CH3CO2H

118 oC

Ethanol

CH3CH2-OH

78 oC

Ethylamine

CH3CH2-NH2

17 oC

Ethane

CH3CH3

-89 oC

Q/ Explain the reason for the difference in b.p. of the above compounds.

Factors affecting B.P.

 A solution has a higher boiling point than a pure

solvent .  The b.p. of pure H2O is 100 oC, but that b.p. can be elevated by adding a solute such as a salt. Δ Tb = Kb Cm where Kb is called the boiling-point elevation constant

The reason for elevating b.p. is: 

the number of solvent molecules at the surface of the solution is less than for pure solvent. The surface molecules can be considered “diluted” by the less volatile particles of the solute  The rate of exchange between solvent in the solution and the air above it is lower (vapor pressure of the solvent is reduced)

 A lower pressure means that a higher temperature is

necessary to boil the water in the solution , hence boiling point elevation  Conversely, adding common salt to water will lower its freezing point.

Ionic vs. covalent compounds 1413 oC Na+ Cl- Na+ Cl- ClCl- Na+Na+Cl-Na+ Na+

Boiling occurs when a temp. is reached at which the thermal energy of the particles ix great enough to overcome the attractive forces that hold them in liquid

-161.5 oC

In the liquid state of a covalent compound, the weak intermolecular interactions are more are more easily overcome and boiling occurs at much lower temp.

Apparatus A small-scale simple distillation apparatus (sample size ~ 10ml)

Miniscale boiling point determination sample size ~0.5 – 1 ml

Procedure