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Goals1. What are alkenes, alkynes, and aromatic compounds?
Be able to recognize the functional groups in these three families of unsaturated organic compounds and give examples of each.
2. How are alkenes, alkynes, and aromatic compounds named? Be able to name an alkene, alkyne, or simple aromatic compound from its structure, or write the structure, given the name.
3. What are isomers? Be able to recognize and draw constitutional isomers.
4. How are organic molecules drawn?
Be able to convert between structural formulas and condensed or line structures.
5. What are alkanes and cycloalkanes, and how are they named?
Be able to name an alkane or cycloalkane from its structure, or write the structure, given the name.
• What are the general properties and chemical reactions of alkanes?
Be able to describe the physical properties of alkanes and the products formed in the combustion and halogenation reactions of alkanes.
• STEP 1: Name the parent compound. Find the longest chain containing the double or triple bond, and name the parent compound by adding the suffix -ene or -yne to the name for the main chain. If there is more than one double or triple bond, the number of multiple bonds is indicated using a numerical prefix.
• STEP 2: Number the carbon atoms in the main chain so that those with multiple bonds have the lowest index numbers possible. Begin numbering at the end nearer the multiple bond. If the multiple bond is an equal distance from both ends, begin numbering at the end nearer the first branch point.
• Cycloalkenes are quite common. The double-bonded carbon atoms in substituted cycloalkenes are assigned index numbers of 1 and 2 so as to give the first substituent the next lowest possible index number.
• STEP 3: Write the full name. Assign numbers to the branching substituents, and list the substituents alphabetically. Use commas to separate numbers and hyphens to separate words from numbers. Indicate the position of the multiple bond in the chain by giving the number of the first multiple-bonded carbon. If more than one double bond is present, identify the position of each and use the appropriate name ending.
• An addition reaction is a general reaction type in which a substance X—Y adds to the multiple bond of an unsaturated reactant to yield a saturated product that has only single bonds.
• An elimination reaction is a general reaction type in which a saturated reactant yields an unsaturated product by losing groups from two adjacent atoms.
The Chemistry of Vision and Color• Vitamin A, an important biological alkene, is critical to the ability to see.
• A vitamin is an organic molecule required in trace amounts obtained through diet.
• Beta-carotene is a purple-orange alkene that provides our main dietary source of vitamin A. The enzymatic conversion of beta-carotene to vitamin A takes place in the small intestine.
• In the eye, vitamin A is oxidized to retinal, which undergoes cis–trans isomerization of its C11–C12 double bond to produce 11-cis-retinal.
• Reaction with the protein opsin then produces the light sensitive substance rhodopsin.
• The eye has two kinds of light-sensitive cells, rod cells and cone cells.
• When light strikes the rod cells, cis–trans isomerization of the C11–C12 double bond occurs and 11-trans-rhodopsin, also called metarhodopsin II, is produced. This causes a nerve impulse to be sent to the brain where it is perceived as vision.
• Metarhodopsin II is then changed back to 11-cis-retinal for use in another vision cycle.
The Chemistry of Vision and Color (Continued)• Whenever there is conjugation in a molecule, a delocalized region of
electron density is formed that is capable of absorbing light.
• Organic compounds with small numbers of delocalized electrons, such as benzene, which has three conjugated double bonds, absorb in the ultraviolet region of the electromagnetic spectrum.
• Compounds with longer stretches of alternating double and single bonds (10 or more) absorb in the visible region.
• The color that we see is complementary to the color that is absorbed; the plant pigment, cyanidin, absorbs greenish-yellow light and thus appears reddish-blue.
• Observed and absorbed colors are complementary. Thus, if a substance absorbs red light, it has a green color.
Addition of Cl2 and Br2 to Alkenes and Alkynes: Halogenation
• Alkenes and alkynes react with halogens to yield 1,2-dihaloalkane addition products:
• The reaction between ethylene and chlorine provides the starting point for the manufacture of PVC plastic.
• Bromine (Br2) can be used as a test for multiple bonds and unsaturation of fats: – A few drops are added to a sample of an unknown compound. – Immediate disappearance of the color reveals the presence of a
multiple bond, as the bromine reacts to form a colorless dibromide.
• The addition of HBr to 2-methylpropene is an example.
• Only one of the two possible addition products is obtained. 2-methylpropene could add HBr to give 1-bromo-2-methylpropane, but it does not; it gives only 2-bromo-2-methylpropane as the major product.
• Markovnikov’s rule—In the addition of HX to an alkene, the major product arises from the H attaching to the double-bond carbon that has the larger number of H atoms directly attached to it, and the X attaching to the carbon that has the smaller number of H atoms attached.
• If an alkene has equal numbers of H atoms attached to the double-bond carbons (a symmetrically-substituted double bond), both possible products are formed in approximately equal amounts.
13.6 Reactions of Alkenes and AlkynesHow Addition Reactions Occur
• Detailed studies show that alkene addition reactions take place in two distinct steps.– In the first step, two electrons move from the C=C double bond to form
a C–H bond.
– In the second step, X2 uses two electrons to form a bond to the positively charged carbon.
• Almost all organic reactions occur between an electron-rich species and an electron-poor species.
• In the first step, the electron-rich alkene reacts with the electron-poor acid. The C=C bond partially breaks, and two electrons move from the double bond to form a new single bond.
• The remaining double-bond carbon has only six electrons in its outer shell and bears a positive charge.
• Carbons that possess a positive charge, or carbocations, are highly reactive. As soon as this carbocation is formed, it immediately reacts with the halogen to form a neutral product.
13.6 Reactions of Alkenes and AlkynesHow Addition Reactions Occur
• The more carbons attached to a carbocation, the more stable it will be, making its formation more favorable.
• Tertiary (3°) carbocations are more stable than secondary (2°) carbocations.
• Secondary (2°) carbocations are much more stable than primary (1°) carbocations.
• Primary (1°) carbocations are so unstable that they almost never form.
• This provides the scientific basis for Markovnikov’s rule: the major product arises because the intermediate it is derived from is more stable than any other intermediate.
• A description of the individual steps by which old bonds are broken and new bonds are formed in a reaction is called a reaction mechanism.
• The reaction begins by addition of an initiator to an alkene; this results in the breaking of one of the bonds making up the double bond.
• A reactive intermediate that contains an unpaired electron (known as a radical) is formed in this step, and this reactive intermediate adds to a second molecule.
• This produces another reactive intermediate, which adds to a third alkene molecule.
• The result is continuous addition of one monomer after another to the end of the growing polymer chain.
• Polymers formed in this way are called chain-growth polymers. The basic repeating unit is enclosed in parentheses, and the subscript n indicates how many repeating units are in the polymer.
13.8 Aromatic Compounds and the Structure of Benzene
• The double-bond electrons are free to move over the entire ring.
• Each carbon–carbon bond is intermediate between a single bond and a double bond. The name resonance is given to this phenomenon.
• Simple aromatic hydrocarbons are nonpolar, insoluble in water, volatile, and flammable.
• Several aromatic hydrocarbons have biological effects: benzene has been implicated as a cause of leukemia, and dimethyl-substituted benzenes are central nervous system depressants.
• Disubstituted benzenes are unique in that the relational descriptors o- (ortho), m- (meta), and p- (para) may be used in place of 1,2-, 1,3-, and 1,4-.
• The terms ortho-, meta- or para- are then used as prefixes.
• Many substituted aromatic compounds have common names in addition to their systematic names. – Methylbenzene is toluene.– Hydroxybenzene is phenol. – Aminobenzene is aniline.
• Frequently, these common names are used with o- (ortho), m- (meta), or p- (para) prefixes.
5. What are the typical reactions of alkenes, alkynes, and aromatic compounds?
• Alkenes and alkynes undergo addition reactions to their multiple bonds.
• Addition of hydrogen to an alkene (hydrogenation) yields an alkane product.
• Addition of Br2 or Cl2 (halogenation) yields a 1,2-dihaloalkane product.
• Addition of HBr and HCl (hydrohalogenation) yields an alkyl halide product.
• Addition of water (hydration) yields an alcohol product.
• Markovnikov’s rule predicts that in the addition of HX or H2O to a double bond, the H becomes attached to the carbon with more H’s and the X or OH becomes attached to the carbon with fewer H’s.
• Aromatic compounds are unusually stable but can be made to undergo substitution reactions, in which one of the ring hydrogens is replaced by another group.
• Among these substitutions are nitration, halogenation, and sulfonation.