Quiz 5 Study Guide Chemistry 231. Number of Isomers. Trends. * As number of Carbons increase in the same family the following occurs:

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1 Quiz 5 Study Guide Chemistry 231 Isomers: compounds that have the same chemical formula but different structures Number of Alkane Isomers Formula Number of Isomers C 6 H 14 5 C 7 H 16 9 C 8 H C 9 H C 10 H C 15 H 32 4,347 C 20 H ,319 C 30 H 62 4,111,846,783 * Table 3.2 McMurry 7 th ed. Pg 81, See McMurry 6 th ed. Pg 76 Trends * As number of Carbons increase in the same family the following occurs: - Increase in Molar Mass - Density as a liquid Increases (density overall increases) - Increase in Melting Point - Increase in Boiling Point - Increase in Heating value (ex. Hexane will have a hotter flame that butane) - Decrease in Solubility in aqueous system not very soluble anyway Decrease in Solubility in non-aqueous system not very soluble anyway - Decrease in Vapor pressure * Within the same family; - Molar mass increases: states of matter change from gas, liquefiable gas, liquid, and solid * Some families start as a liquid (ex. Carboxylic acids e.g., Formic acid and Acetic acid)

2 Branching * When isomers form it is due to branching of the molecule (ex. Butane has 2 isomers, Pentane has 3 isomers) * Branching does not change vapor density (it is independent) - Calculate vapor density by taking: Molar Mass 22.4 L - 1 mole of any ideal gas = 22.4 STP (0 Celsius, 1atm) - 1atm = in. Hg = 760 mm Hg = 76 cm Hg = 760 torr = psia * Within the Alkane family, as branching increases the alkane s boiling point is lowered. Examples: Alkane Boiling Point - Pentane (no branches) 36.1 degrees C - Iso-pentane (one branch) degrees C (2-methylbutane) - Neo-pentane (two branches) 9.5 degrees C (2, 2-dimethylpropane) - Octane (no branches) degrees C - Iso-octane (three branches) 99.3 degrees C (2, 2, 4-trimethylpentane) * Branched-chain alkanes are lower-boiling because they are more nearly Spherical than straight chain alkanes, have smaller surface areas, and consequently have smaller dispersion forces. *See McMurry 6 th ed. Pg 88, or McMurry 7 th ed. Pg 52 * Alkanes show regular increases in both boiling point and melting point as molecular weight increases. This affect is due to weak dispersion forces between molecules. Only when sufficient energy is applied to overcome these forces does the solid melt or liquid boil Dispersion forces increase as molecule size increases, this accounts for the higher melting and boiling points. *See McMurry 6 th ed. Pg 87, or McMurry 7 th ed. Pg 92

3 Physical properties of n-alkanes Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane * Alkanes are occasionally referred to aliphatic compounds. This comes from the Greek word aleiphas, meaning fat. In organic chemistry today it means loosely saturated. Physical properties of alkenes Ethylene Propylene butene pentene hexene heptene octene *Hydrocarbons that contain two or more double bonds are known as alkadienes, alkatrienes, and alkatetraenes. The suffix denotes the number of double bonds. Ex. CH 2 =C=C=CH-CH 3 (1, 2,3-pentatriene)

4 Physical properties of the butylenes Isobutylene butene Trans-2-butene Cis-2-butene Physical properties of cyclic aliphatic hydrocarbons Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cycloheptane Cyclooctane Physical properties of aromatic (arenes) hydrocarbons Benzene Toluene o-xylene m-xylene p-xylene

5 Physical properties of hexane isomers n-hexane methylpentane methylpentane (isohexane) 2, 3-dimethylbutane , 2-dimethylbutane (neohexane) Hydrocarbons Hydocarbons Aliphatics Aromatics (arenes) Alkanes Alkenes Alkynes Cyclic aliphatics * Vapor Pressure is the pressure above a liquid at equilibrium. * If the pressure on a liquid is reduced it will boil at lower temperature than its boiling point. * Vapor pressure increases with temperature non-linearly.

6 Gasoline Fractional distillation is used to refine crude oil into three principle cuts according to their boiling points. -Straight run gasoline (boiling point C) -kerosene (boiling point C) -heating oil or diesel fuel (boiling point C) After reducing the pressure further distillation yields: -lubricating oils and waxes This leaves undistillable tarry residue of asphalt. * Straight run gasoline contains a large number of straight chain alkanes. Therefore it is a poor fuel because it causes engine knock. In order to create a higher quality fuel catalytic cracking is used to break down larger molecules of kerosene (with high boiling point) into smaller molecules which are used for gasoline * See McMurry 6 th ed. Pgs or McMurry 7 th ed. Pgs *Octane number of fuel is the measure by which the antiknock properties of fuel are judged. Octane Rating - Measure of the autoignition resistance of gasoline and other fuels used in spark ignition internal combustion engines. * In a gasoline engine a high octane rating is desired. * In a diesel a low octane rating is desirable. Examples of Octane Ratings: - n-octane n-heptane 0 - n-pentane 62 - iso-octane benzene E85 ethanol 105

7 * Interesting information: In winter months butane is added to gas to increase the octane. This is not done in summer because it is too hot and will cause vapor locking and it will evaporate out of the tank. * Natural Gas liquid (NGL) from natural gas wells Examples: - Propane boiling pt -44 degrees F (used in wasp sprays) - Isobutane boiling pt 10 degrees F (used in under arm deodorant) - n-butane boiling point 31 degrees F (used in glass perfume bottles) Liquefied petroleum Gas (LPG) comes from petroleum crude and is cat cracked (catalyst)