Chem 6AL Midterm 1

What are intermolecular forces?

• weaker than covalent bonds

• the attraction between molecules

• London dispersion, dipole dipole, H bonding

London Dispersion

temporary induced dipoles

• the induced can induce other adjacent molecules

• higher area contact = stronger LD = more E needed

Dipole - Dipole

• stronger than LD forces

• attraction b/w polar molecules

H-Bonding

strong DD attraction for H bonded to N,O,F with its lone pair

• primary bonds are stronger in DD and HB

Noncovalent interactions (IM) affect properties and purification

• boiling point

• melting point

• solubility

• Melting point range

• Distillation

Boiling point

• depends on the strength of attractive forced b/w molecules

• Strong forces = high BP = inc molec weight

• branching lowers BP by dec area of contact

Melting point

• Molec weight inc = inc melting point

• influenced by packing of the crystal lattice

• tight and organized packing requires more E to break it apart = higher melting point

• impurities lowers the mp and broadens its range by disrupting the lattice = less E needed

• odd carbon count = lower mp

Melting Point technique

• pack dry powder in capillary tube

• heat slowly towards its expected mp

• record the first sign of melting and when its finished (this is the mp range)

Solubility

like dissolves like

• polar solutes dissolve in polar solvents by HB, DD, Ion dipole

• nonpolar solutes dissolve in nonpolar solvents by LD

Distillation

BP differences = IM differences

• boiling liquids and collecting its vapor

• high BP distills later

• low BP distills first

Distillation - Ideal liquids and Raoult’s Law

• IM are similar

• obey the law to be ideal

Raoult’s Law

PA=(XA)(P_⁰A)

• PA = partial vapor pressure of A

• XA = mole fraction of A in the liquid

• PA∘ = vapor pressure of pure A

Simple Distillation

• BP differs by a greater amount than 60 degrees

• fewer theoretical plates

Fractional Distillation

• BP differs by an amount less than 60 degrees

• More theoretical plates = better separation

• similar IM

Temp Composition Diagram

• liquid comp curve = lower curve

• vapor comp curve = higher curve

Vapor Pressure

Higher VP = lower BP

Azeotropes

• boils at constant temp

• vapor composition = liquid composition

• cant be separated by distillation

Freq of IR

Freq: determined by bond strength and reduced mass

• stronger bonds and lighter atoms = higher freq

• High E = high freq = high wv# = shorter wavelength

Intensity of IR

Intensity: determined by change in dipole moment during vibration

• Big dipole change = strong peak

• small dipole change = weak peak

Stretches

• change bond length along line of bond

• strong and sharp

• High E = High wv #

bend

• doesn’t occur along bond length

• weak and broad

• low E = Low wv#

Aldehyde / ketone / C=O abs band

~1720 cm^-1

OH abs band

~3600 - 3300 cm^-1

• broad

• tongue shape

NH abs band

~3400 cm^-1

• sharp peak

• primary = 2 peaks

• secondary = 1 peak

• tertiary = 0 peaks

Carboxy A abs band

~3200 - 2500 cm^-1

• broad, wide

• mountain shape

C≡C

~2200 cm⁻¹

C≡N

~ 2250 cm⁻¹

C=C

~ 1650 cm⁻¹

C=N

~ 1650 cm⁻¹

sp CH

~3300 cm⁻¹

sp2 CH

~3150 - 3020 cm⁻¹

sp3 CH

~ 2950 cm⁻¹