Chapter Menu Hydrocarbons Section 21.3Section 21.3 Alkenes and Alkynes Exit Click a hyperlink or folder tab to view the corresponding slides.

Section 21-3 Alkenes and Alkynes Compare the properties of alkenes and alkynes with those of alkanes. hormone: chemical produced in one part of an organism and transported to another part, where it causes a physiological change Describe the molecular structures of alkenes and alkynes. Name an alkene or alkyne by examining its structure. Draw the structure of an alkene or alkyne by analyzing its name.

Section 21-3 Section 21.3 Alkenes and Alkynes (cont.) alkene alkyne Alkenes are hydrocarbons that contain at least one double bond, and alkynes are hydrocarbons that contain at least one triple bond.

Section 21-3 Alkenes Unsaturated hydrocarbons that contain at least one or more double covalent bonds between carbon atoms are called alkenes.alkenes

Section 21-3 Alkenes (cont.) Alkenes are named in much the same way as alkanes. Alkenes end in –ene. When four or more carbon atoms are present, specify the location of the double bond.

Section 21-3 Alkenes (cont.) When naming branched-chain alkenes, follow the same rules as for alkanes, with two exceptions. −The parent chain is always the longest chain that contains double bond, whether it is the longest chain or not. −The position of the double bond, not the branches, determine the numbering. −Use a prefix to designate the number of double bonds.

Section 21-3 Alkenes (cont.) Alkenes are nonpolar and have low solubility in water. Alkenes are more reactive than alkanes because the double bond increases electron density between the two carbon atoms, providing a good site for chemical reactivity.

Section 21-3 Alkynes Unsaturated hydrocarbons that contain one or more triple bonds between carbon atoms are called alkynes.alkynes

Section 21-3 Alkynes (cont.) Straight-chain and branched-chain alkynes are named in the same way as alkenes, except the ending is –yne.

Section 21-3 Alkynes (cont.) Alkynes have physical and chemical properties similar to alkenes but are generally more reactive because the triple bonds cause even larger electron densities than double bonds.

Physical Properties Physical properties of the alkenes and alkynes are quite similar to those of alkanes –Nonpolar –Not soluble in water –Highly soluble in nonpolar solvents Boiling points rise with molecular weight 11.1 Structure and Physical Properties

11.5 Reactions Involving Alkenes and Alkynes There are two kinds of reactions typical of alkenes: –Addition: two molecules combine to give one new molecule –Redox: oxidation and reduction The two classes are not always mutually exclusive

Addition: General Reaction A small molecule, AB, reacts with the pi electrons of the double bond The pi bond breaks and its electrons are used to bond to the A and B pieces Some additions require a catalyst 11.5 Reactions Involving Alkenes and Alkynes

Types of Addition Reactions 1.Symmetrical: same atom added to each carbon Hydrogenation - H 2 (Pt, Pd, or Ni as catalyst) Halogenation - Br 2, Cl 2 2.Unsymmetrical: H and another atom are added to the two carbons Hydrohalogenation - HCl, HBr Hydration - H 2 O (requires strong acid catalyst e.g., H 3 O +, H 2 SO 4, H 3 PO 4 ) 3.Self-addition or polymerization 11.5 Reactions Involving Alkenes and Alkynes

Hydrogenation: Addition of H 2 Hydrogenation is the addition of a molecule of hydrogen (H 2 ) to a carbon-carbon double bond to produce an alkane The double bond is broken Two new C-H bonds result Platinum, palladium, or nickel is required as a catalyst Heat and/or pressure may also be required 11.5 Reactions Involving Alkenes and Alkynes

Halogenation: Addition of X 2 Halogenation is the addition of a molecule of halogen (X 2 ) to a carbon-carbon double bond to produce an alkane The double bond is broken Two new C-X bonds result Reaction occurs quite readily and does NOT require a catalyst Chlorine and bromine are most often the halogen added 11.5 Reactions Involving Alkenes and Alkynes

Bromination of an Alkene 11.5 Reactions Involving Alkenes and Alkynes Left beaker contains bromine, but no unsaturated hydrocarbon Right beaker contains bromine, but reaction with an unsaturated hydrocarbon results in a colorless solution

Hydration A water molecule can be added to an alkene –The addition of a water molecule to an alkene is called hydration Presence of strong acid is required as a catalyst Product resulting is an alcohol 11.5 Reactions Involving Alkenes and Alkynes

Markovnikov’s Observation Dimitri Markovnikov (Russian) observed many acid additions to C=C systems He noticed that the majority of the hydrogen went to a specific end of the double bond He formulated an explanation 11.5 Reactions Involving Alkenes and Alkynes

Markovnikov’s Rule When an acid adds to a double bond –The H of the acid most often goes to the end of the double bond, which had more hydrogens attached initially H-OH H-Cl H-Br 11.5 Reactions Involving Alkenes and Alkynes

Hydration of Alkynes Hydration of an alkyne is a more complex process –The initial product is not stable Enol produced – both an alkene and an alcohol Product is rapidly isomerized –Final product is either Aldehyde Ketone 11.5 Reactions Involving Alkenes and Alkynes

Hydrohalogenation An alkene can be combined with a hydrogen halide such as HBr or HCl The reaction product is an alkyl halide Markovnikov’s Rule is followed in this reaction 11.5 Reactions Involving Alkenes and Alkynes

Reaction Schematic Alkene Hydrogenation Hydration Halogenatio n Hydrohalogenation + H 2 Pt, Pd, or Ni + H 2 O acidic + X 2 adds easily + HX

Summary of Reactions 1. Addition Reactions of Alkenes a. Hydrogenation b. Hydration c. Halogenation d. Hydrohalogenation 2. Addition Polymers of Alkenes 3. Reactions of Benzene a. Halogenation b. Nitration c. Sulfonation

A.A B.B C.C D.D Section 21-3 Section 21.3 Assessment Which of the following is generally the most reactive? A.cycloalkanes B.alkanes C.alkenes D.alkynes

A.A B.B C.C D.D Section 21-3 Section 21.3 Assessment What is the name of a straight-chain hydrocarbon with six carbon atoms and a triple bond between the second and third carbon atoms? A.2-hexene B.3-hexene C.2-hexyne D.3-hexyne

Study Guide 3 Alkenes and Alkynes Key Concepts Alkenes and alkynes are hydrocarbons that contain at least one double or triple bond, respectively. Alkenes and alkynes are nonpolar compounds with greater reactivity than alkanes but with other properties similar to those of alkanes.