Benzene

 

Benzene

Chemical formula	C6H6
OTP appearance	clear liquid
Molar Mass(g/mol)	78
Density(g/cc)	0.8765
Melting Point(°C)	5.5
Boiling Point(°C)	80.1
Fuel Value(MJ/kg)	40.170
Solubility in water(g/L)	18
NFPA 704	3 2 0

Uses

Primary

• Industrial chemical
• Precursor to all aromatic carbon compounds
• Feedstock for nitrobenzene, aniline, toluene, phenol, ethyl benzene etc.

Natural occurrence

• Rarely, small amounts are emitted from volcanic/petroleum eruptions.
• Many components of essential oils (thymol, p-cymene, carvacrol) contain benzene rings with short alkyl and hydroxyl attachments

Hazards

• Highly flammable
• Carcinogen

Production

Synthesis

from ethanol

1. ethanol to ethylene (via sulfuric acid)
2. ethylene to dibromoethane (via hydrogen bromide)
3. dibromoethane to acetylene (via potassium hydroxide)
4. acetylene to benzene (as below)

from acetylene

Take 3 molecules of acetylene, and add more energy.

uncatalyzed

Heat acetylene in an oxygen-free environment, trimerizing it to benzene.

3 C₂H₂

{

→

400°C}

C₆H₆

This method produces a legion of side products, including carbon and hydrogen gas, as well as other tarry materials. The reaction products then need to be separated by distillations.

catalyzed

The reaction temperature can be lowered substantially by sufficiently active catalysts,

from trimethyl benzene

This is an extremely inefficient synthesis of benzene, and included only so there is a synthetic path. N.B. Propanone and trimesic acid are soluble in ethanol while trimethyl benzene is not.

1. Slowly add dilute nitric acid to trimethyl benzene and mix thoroughly to produce trimesic acid (powder, mp near 370°C; 210g/mol)

   5 C₉H₁₂ + 18 HNO₃ → 5 C₉H₆O₆ + 24 H₂O + 9 N₂

   ratios 25g tmb : 27g acid dissolved in ≥250g of water

2. Thoroughly mix powders of trimesic acid and a molar excess of sodium hydroxide or calcium oxide
3. Dry distill the mixture

   C₉H₆O₆(s) + 3 CaO(s)

   {

   →

   heat}

   C₆H₆(v) + 3 CaCO₃(s)

   C₉H₆O₆(s) + 6 NaOH(s)

   {

   →

   heat}

   C₆H₆(v) + 3 Na₂CO₃(s) + 3 H₂O(v)

4. The distillate is benzene