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CHAPTER 22Organic and Biological Molecules
Organic Chemistry
Organic chemistry is all about carbon compounds, the element of living organisms.
To name organic compounds, you will use prefixes to indicate the number of atoms present.
Meth = one Eth = two Prop = three But = four Pent = five Hex = six Hept = seven Oct = eight Non = nine Dec = ten
Boiling Points of Organic Molecules
Boiling points reflect the strength of forces between molecules. The more they stick together, the more energy it will take to blast them into the atmosphere as gases.
The relative strength of the four intermolecular forces is: Ionic >hydrogen bonding>dipole dipole>van der Waals (London dispersion) forces.
Boiling points increase as the number of carbons increase.
Branching decreases boiling point.
Size does matter
Among molecules with roughly similar molecular weights, the boiling points will be determined by the functional groups present.
Bigger molecules have higher boiling points. When the number of electrons increase, the temporary dipole attractions increase which then increases the dispersion forces. So the bigger the molecule, the “stickier” they are to each other.
Shape matters Long thin molecules can develop bigger
temporary dipoles due to electron movement more so than short fat ones containing the same number of electrons. The top molecule is a 2-methylpropane and the bottom is a butane molecule. Which one would have the higher boiling point?
Molecules with permanent dipole –dipole attractions will have a higher boiling point then those with temporary fluctuating dipoles. Compare fluoromethane and ethane. Both have the same number of electrons and are the same size, but one has a permanent dipole-dipole attraction. Which one has the higher boiling point?
Ethane Fluoromethane
Predict B.P. for CO2 and SO2
Saturated hydrocarbons
Carbon and hydrogen compounds with single bonds:
Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane Formula: CnH2n+2
Properties of alkanes
Unsaturated hydrocarbons
These have multiple bonds somewhere in the alkane chain.
An example is ethene, also known as ethylene: C2H4
These typically readily undergo rapid addition reactions.
Structural isomerism
The larger molecules can form different structures.
To name them, you first look for the longest continuous carbon chain. In this case its propane.
Then you need to indicate how many chlorines there are and where they are located on the carbon chain.
First one is a 1-chloropropane. Second one is a 2-
chloropropane.
Isomers with carbon substituent groups
(11:31) Octane 2-Methylheptane 3-Methylheptane 4-Methylheptane 2,2-Dimethylhexane 2,3-Dimethylhexane 2,4-Dimethylhexane 2,5-Dimethylhexane 3,3-Dimethylhexane 3,4-Dimethylhexane 3-Ethylhexane 2,2,3-Trimethylpentane 2,2,4-Trimethylpentane 2,3,3-Trimethylpentane 2,3,4-Trimethylpentane 2-Methyl-3-ethylpentane 3-Methyl-3-ethylpentane Tetramethylbutane
Aromatic Hydrocarbons
These rings typically do not undergo addition reactions like other unsaturated hydrocarbons. Instead, they tend to undergo substitution reactions where hydrogens are replaced by other atoms.
Polymers
Polymers are large, usually chainlike molecules that are built from smaller molecules called monomers. This is where plastic comes from…
Common Synthetic Polymers
Monomer Polymer Uses
Ethylene Polyethylene Plastic piping, bottles, electrical insulation, toys
Propylene Polypropylene Film for packaging, carpets, lab wares, toys
Vinyl chloride Polyvinyl chloride (PVC) Piping, siding, floor tile, clothing, toys
Acrylonitrile Polyacrylonitrile Carpets, fabrics
Tetrafluoroethylene
Teflon Cooking utensils, electrical insulation, bearings
Styrene Polystyrene Containers, thermal insulation, toys
Butadiene Polybutadiene Tire tread, coating resin
Butadien and styrene
Styrene-butadiene rubber
Synthetic rubber
Natural polymers
Proteins are natural polymers where the building blocks are amino acids.
Through condensation reactions, the amide group forms polypeptide chains that eventually yield a protein.
Common Mistakes to Avoid
When writing organic formulas, make sure that every carbon has four bonds.
When naming alkanes, make sure to number the carbon chain so the sum of all location numbers is as small as possible.
When naming branched alkanes, be sure to consider the branches when finding the longest carbon chain.
In naming identical substituents on the longest carbon chain, be sure to use repeating location numbers, separated by commas (2,2-dimethyl).
Free-Response Question
The alkane hexane, C6H14, has a molecular mass of 86.17 g/mol.
Like all hydrocarbons, hexane will burn. Write a balanced chemical equation for the complete combustion of hexane. This reaction produces gaseous carbon dioxide, CO2 and water vapor, H2O. (you knew that!)
2C6H14 + 19 O2 12 CO2 + 14 H2O Give yourself 2 points for the answer shown
above or for the coefficients 1, 9/2, 6, and 7. Give yourself 1 point if you have one or more, not
all, of the elements balanced.
The combustion of 10.0 g of hexane produces 487 kJ. What is the molar heat of combustion (ΔH) of hexane?
(-487 kJ/10.0g)(86.17 g hexane/mol) = -4.20 x 103 kJ/mol
Give yourself 2 points for the above setup and correct answer (this requires a negative sign in the answer). If the setup is partially correct, give yourself 1 point.
Determine the pressure exerted by the carbon dioxide formed when 5.00 g of hexane is combusted. Assume the carbon dioxide is dry and stored in a 20.0 L container at 27oC.
P = nRT/V (5.00 g C6H14 ) (1 mol C6H14/86.17 g C6H14)(12 mol
CO2/2 mol C6H14) = 0.348 mol CO2
The mole ratio should match the one given in your balanced equation. You will not be penalized again for an incorrectly balanced equation.
You will lose a point if you do not include a hexane to CO2 conversion.
R = 0.08206 L· atm/mol · K T = 27oC + 273 = 300.0 K V = 20.0 L
P = (0.3481 mol CO2)(0.08206 L· atm/mol · K)(300.0 k)/20.0 l = 0.429 atm
Give yourself 2 points for the correct setup and answer. Give yourself 1 point if you did everything correctly, except the mole ratio or the Kelvin conversion.
Hexane, like most alkanes, may exist in different isomeric forms. The structural formula of one of these isomers is pictured below. (not really, there is no Internet to get the picture so assume it’s a straight-chain, n-alkane) Draw the structural formula of any two other isomers of hexane. Make sure all the carbon atoms and hydrogen atoms are shown.
Give yourself 1 point for each correct answer, with a 2-point maximum. There are no bonus points for additional answers.
Total your points. The maximum 8 points. Subtract one point if all your answers do not have the correct number of significant figures.
