9. Hydrocarbons

Question 1

Organic compounds involve carbon + …..

Answer 1

• mostly hydrogen, oxygen + nitrogen
• with a little sulfur, flourine, chlorine, bromine, iodine + phosphorous
• almost all important chemicals in living systems are organic compounds, including carbohydrates, lipids, proteins, enzymes, nucleic acids (DNA, RNA), hormones + vitamins
• bonding is almost entirely covalent
• most have low melting point < 360oC
• most insoluble in water
• most soluble in organic solvents

aqueous solutions of organic compounds do not conduct electricity

• almost all burn + decompose
• reactions are usually slow
• living organisms produce organic compounds by biosynthesis

Question 2

Typical bonding and structure of organic compounds

Answer 2

• carbon normally forms four covalent bonds + has no unshared electron pairs
• nitrogen normally forms three covalent bonds + has one unshared electron pair
• oxygen normally forms two covalent bonds + has two unshared electron pairs
• hydrogen normally forms one covalent bond + has no unshared electron pairs
• a halogen - flourine, chlorine, bromine + iodine - normally forms one covalent bond + has three unshared electron pairs
• 4 regions of electron density = tetrahedral distribution = 109.5° bond angle
• 3 regions of electron density = trigonal planar distribution = 120° bond angle
• 2 regions of electron density = linear distribution = 180° bond angle

Question 3

Resonance

Answer 3

When the actual structure of a molecule or polyatomic ion cannot be adequately described by a single Lewis structure, it is instead represented as a composite of two or more Lewis structures called ‘resonance structure’. Individually, each resonance structure does not accurately describe the bonding in a species, however, the combination of all resonance structures produces a ‘resonance hybrid’ which depicts the actual structure

Question 4

Isomers

Answer 4

• compunds that have the same molecular formula, but differ in the way their atoms are arranged, either structurally or geometrically
• usually have differing physical + chemical properties

Question 5

Constitutional Isomers

Answer 5

• also known as ‘structural isomers’
• differ in the sequence in which their atoms are bonded together
• when a reaction that can potentially yield two or more structural isomers actually produces only one, ar a predominance of one, the reaction is said to be ‘regioselective’

Question 6

Stereoisomers

Answer 6

• also known as geometrical isomers
• same connectivity of atoms, but a different arrangement in space
• stereoisomers whose molecules are mirror reflections of each other are ‘enantiomers’
• stereoisomers whose molecules are not mirror reflections of each other are ‘diastereomers
• stereoisomers in which two groups are located on the same side of a central imaginary line in the molecule or ion are ‘cis-isomers’. They are diastereomers
• stereoisomers in which two groups are located on opposite sides of a central imaginary line in the molecule or ion are ‘trans-isomers’. They are diastereomers
• stereocentre is a tetrahedral atom, most commonly carbon, at which exchange of two groups produces a steroisomer

Question 7

Hydrocarbons

Answer 7

• compounds containing only carbon and hydrogen
• two main categories: aliphatic + aromatic

Question 8

Aliphatic Hydrocarbons

Answer 8

• include straight - chain, branched - chain + cyclic compounds
• subdivided into saturated + unsaturated
• saturated hydrocarbons contain only carbon - carbon single bonds
• unsaturated hydrocarbons contain at least one carbon - carbon double or triple bond

Question 9

Aromatic Hydrocarbons

Answer 9

• contain highly stable rings of carbon atoms

Question 10

Line angle formula

Answer 10

• an abbreviated way to draw structural formulas in which each vertex + line terminus represents a carbon atom + each line represents a bond
• used particularly for hydrocarbons