Organic ChemActivity
70 Chemistry (I)
(Yeah, carbon is OK, its always willing to share!)
Information Share indeed! Carbons position in the periodic table makes it very unlikely that a carbon atom would either gain or lose four electrons to give the respective tetranegative or tetrapositive ion. Either would simply require too much energy. But, carbon would still like to obtain an octet. This can be accomplished through covalent bonding, that is, the sharing of pairs of electrons.
What makes the covalent bonding of carbon interesting (and very practical!) is the tendency of carbon to covalently bond with both other carbon atoms and hydrogen atoms. As the example above shows, carbon is tetravalent - it forms four bonds. Model 1: Hydrocarbons contain only carbon and hydrogen. In your brief study of organic chemistry and polymers, you will need an introductory knowledge of three types of hydrocarbons (molecules containing only hydrogen and carbon): alkanes: hydrocarbons having only have single bonds between carbons; four examples are shown below:
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Ch em Activ ity 70
Org anic Chemistry (I)
alkenes: hydrocarbons containing at least one carbon-carbon double bond; note these examples and compare them to the alkanes above:
aromatic hydrocarbons = a special type of alkene containing alternating carbon-carbon single and double bonds in a ring; the most common aromatics are based on benzene:
Critical Thinking Questions 1. Confirm that ethane, ethene, and benzene all follow the Lewis Octet Rule. (Organic compounds are notorious for forming octets.)
2.
Draw the structure of an alkane containing five carbons. Also give the condensed structural formula as shown in the Model 1 examples.
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3.
Draw a different five-carbon alkane with the same molecular formula. (HINT: dont put all five carbons in a long chain, make a branch.) Compounds that have the same molecular formula but different structures are called isomers.
4.
Draw an isomer of 1-butene. Remember that it must have the formula C 4 H 8 .
5.
Look at the aromatic compounds in Model 1. What do they all have in common?
6.
Trinitrotoluene is an organic compound but not a hydrocarbon because it contains other elements besides hydrogen and carbon (N and O). You know trinitrotoluene by a different name. What is it?
7.
Consider the shapes of these hydrocarbons. They are all drawn flat on the paper but are they really planar? Considering Electron Domain Theory (ChemActivity17), draw ethane and ethene and label the bond angles (90/, or 109.45/, or 120/...).
Information We often draw condensed structural formulas in a way that indicates the three dimensionality of the molecules. We understand that the hydrogen atoms are bonded to the carbons at the angles dictated by the number of electron domains.
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Org anic Chemistry (I)
Critical Thinking Questions 8. Consider the shapes of these hydrocarbons and the electronegativites of carbon and hydrogen. Are hydrocarbons polar (ChemActivity22)? Explain.
9.
What kind of intermolecular forces hold hydrocarbon molecules together in the liquid state (ChemActivity26)? Explain.
10. Rank the following hydrocarbons in order of increasing boiling point. your Explain C 32 H 66 C 8 H 18 C 3 H 8 ranking. CH 4
11. Two of the substances in CTQ10 are gases, one is a liquid, and one is a solid at room temperature. Identify each and explain your reasoning.
12. Do you expect hydrocarbons to be soluble in water (ChemActivity26A)? Explain
Model 2: The Chemistry of Petroleum Products Petroleum products contain a broad range of hydrocarbons found as gases, liquids, and solids beneath the earths surface. Natural gas contains lightweight alkanes such as methane, ethane, propane, butane and pentane (hydrocarbons with 1, 2, 3, 4, and 5 carbons, respectively). Crude oil contains between 50 and 95% heavier hydrocarbons (C 5 to C 20 and
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greater). The fraction of crude oil which we use as gasoline is predominantly composed of C 6 to C 8 hydrocarbons. Fractions with longer carbon chains include greases and asphalt. These compounds are valued as fuels because they all react with oxygen in the air to produce carbon dioxide and water vapor. The reaction of octane is a good example: 2 C 8 H 18 (l + 25 O 2 (g) 16 CO2 (g) + 18 H 2 O(g) )
Critical Thinking Questions 13. Is the combustion reaction in Model 2 a redox reaction? Explain your reasoning.
14. Would you predict that the reaction in Model 2 is endothermic or exothermic? Explain your reasoning.
15. Following the example in Model 2, write a balanced equation for the combustion reaction of ethane, C 2 H 6 .
16. Calculate )H/ for the combustion of ethane. Did you expect this sign and magnitude for )H/? Explain why.
17. Is the combustion of ethane a naturally occurring process? What must you calculate to answer this question? Explain.
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18. The combustion of gasoline is a spontaneous process. Yet, gasoline exposed to air does not burst into flame. Use basic chemical principles to explain this phenomenon (a diagram might even be nice!).
19. Thro...