Benzene, Aromatic Compounds, and Hydrocarbon Properties

Benzene and Aromatic Compounds (Section 9.8)

Definition and Structural Overview

Aromatic Compounds: This class of hydrocarbons contains benzene rings.
Benzene Ring Structure: Represented as a hexagon with a circle inside or alternating double bonds.
Chemical Formula: $C_6H_6$
Phenyl Group: When the benzene ring acts as a substituent on another chain, it is referred to as a phenyl group.

Specific Compounds and Examples

Naphalene: Consists of two benzene rings fused together.
Kylene: A benzene ring with two methyl groups located right next to each other.
Substituted Benzenes (Nomenclature): - Methylbenzene: Also known as Tolu- (Toluene). It consists of a benzene ring attached to a methyl ( $CH_3$ ) group. - Propyl benzen: A benzene ring attached to a propyl ( $CH_2CH_2CH_3$ ) group. - Styrene: A benzene ring attached to an alkene group ( $CH=CH_2$ ). This is represented as $C_6H_5CH=CH_2$ .

Properties of Hydrocarbons (Section 9.9)

Bonding and Composition

Hydrocarbons are composed entirely of C-H (carbon-hydrogen) bonds.

Physical Properties

Density: Typically ranges from $0.6\,g/mL$ to $0.95\,g/mL$ . - They are less dense than $H_2O$ (water), meaning they will form the top layer when mixed with water.
Solubility: Hydrocarbons are hydrophobic. - They do not dissolve in water because they have no $-OH$ groups to form hydrogen bonds ( $H-bond$ ) with $H_2O$ .

Physical State at Room Temperature ( $RT$ ) The state of a hydrocarbon at room temperature depends on the number of carbon atoms (#Cs) in the molecule:

Gaseous ( $g$ ): If there are $1-4$ carbons.
Liquid ( $l$ ): If there are $5-14$ carbons (this also depends on the specific structure).
Solid ( $s$ ): If there are $15$ or more carbons ( $15 \dots$ ).

Physical Constants and Molecular Weight Data

Hydrocarbons generally have relatively low melting points ( $m.p.$ ) and boiling points ( $b.p.$ ). Below is a comparison table of specific hydrocarbons and water ( $H_2O$ ):

Compound	Molec. Weight ( $g/mol$ )	Met.pt ( $^{\circ}C$ )	Boil. pt ( $^{\circ}C$ )
$CH_4$ (C1)	$16.04\,g/mol$	$-182^{\circ}C$	$-162^{\circ}C$
$C_3 H_8$ (C3)	$44.11\,g/mol$	$-188^{\circ}C$	$-42^{\circ}C$
$C_5 H_{12}$ (C5)	$72.17\,g/mol$	$-130^{\circ}C$	$36^{\circ}C$
$C_?$	$160.23\,g/mol$	$-90^{\circ}C$	$98^{\circ}C$
$H_2O$	$18.02\,g/mol$	$0^{\circ}C$	$100^{\circ}C$

Trends Observed:

As molecular weight (MW) increases, the boiling point (bp) increases.
Note: Hydrocarbons are significantly less polar than water, which is evident by comparing the boiling point of $CH_4$ ( $16.04\,g/mol$ , b.p. $-162^{\circ}C$ ) to $H_2O$ ( $18.02\,g/mol$ , b.p. $100^{\circ}C$ ).

Chemical Reactivity and Reactions

Reactivity Overview

Hydrocarbons are relatively inert, meaning they do not react much under normal conditions.

Combustion Reactions

The primary reaction for hydrocarbons is combustion, which releases energy.
General Example (Butane): $2 C_4 H_{10} + 13 O_2 \rightarrow 8 CO_2 + 10 H_2O + \text{energy}$

Comparative Study: Cycloalkanes (S.P 9.19)

Example: Cyclohexane vs. Cyclopentane

Problem: Determine which compound will have the highest boiling point.
Conclusion: Regarding members of the same series, cyclohexane will have the highest bothing of [the two] because it has a larger molecular weight and more carbon atoms than cyclopentane.