Exercise 3: Boiling Point and Melting Point of Organic Compounds (copy)

boiling point

temperature at which the vapour pressure substance is equal to the atmospheric pressure above it

van der waals dispersion forces

weak intermolecular attraction formed by induced polarization. commonly observed among hydrocarbon groups and occurs only within short distances

dipole-dipole interaction

occurs among partially ionic or polar molecules. contain an asymmetrical charge due to the two poles, one end negative while the other end positive

hydrocarbon bonding

formed between partially positive and an adjacent electronegative atom which bears a partial negative charge. it tend to rise boiling point

ionic bonding

takes place between molecules having opposite charges and involves an electrostatic interaction between the two opposite charges

length of the carbon chain

factor that can affect boiling point

boiling point ↑ as the number of carbon atoms ↑

due to more energy that is required to overcome the increased forces of attraction caused by long carbon chains

boiling point ↓ in the presence of branchin

causes the surface area of the molecule to decrease thereby the forces of attraction between individual molecules

boiling point ↓ in the presence of volatile impurities

due to the increase in the vapor pressure of the liquid

melting point

temperate at which a solid compound is directly converted to its liquid statee

pure crystalline

has a sharp melting point and it completely melts within a narrow range of temperature NMT 0.5-1 C

if compound melts over a very narrow range

the compound tested is relatively pure

impurities

usually depresses the melting point and may increase the melting point range

presence of polarity

leads to a higher melting point of compounds

tert-butyl alcohol

82.3

chloroform

61.2

n-hexane

68.7

methanol

64.7

salicylic acid

211.0

benzoic acid

122.4

glucose

146.0

fructose

103.0