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+ Alkenes, Alkynes and Aromatic Structures + Unsaturated Hydrocarbons - consist of carbon based molecules with multiple bonds between the carbons Alkenes- functional group that is characterized by C-C double bonds Alkynes- functional group characterized by C-C triple bonds Aromatics- structures that contain a benzene ring + Characteristics of Alkenes Alkenes have a hybridization of sp2, a molecular geometry of trigonal planar and a bond angle of roughly 120° General formula: They hybridization allows for 3 sp2 orbitals and 1 p orbital, which is why the carbon is still able to have 4 bonds. They are hydrocarbons, so they are nonpolar in nature. Structural isomers exist (same chemical formula but different arrangement of atoms). Geometric isomers also exist (more on this later.) + Naming Alkenes Pretty similar to alkanes…but, The name of the longest carbon chain ends in –ene. You start numbering the carbons in the longest chain closest to the double bond. You must indicate where the double bond is in the compound, so you use the lowest numbered carbon that is attached to the double bond. Example: 2-hexene or hex-2-ene + Branches off of alkene chains Most are the same as alkanes Chloro Bromo Iodo Methyl Ethyl Propyl If you have a C2H3 – group off of the main chain, it is called a vinyl group. An allyl group + Practice Naming and Drawing Alkenes 1) 3-hexene 2) 4-ethyl-2-heptene 3) 3,4-dimethyl-2-pentene 4) 2-cyclohexyl-3-hexene 5) 6) + Multiple Double Bonds in Alkenes Many compounds contain two or more double bonds and are known as alkadienes, alkatrienes, alkatetraenes, and so on, the suffix denoting the number of double bonds. The location of each double bond is specified by appropriate numbers, as illustrated to the right: 1,2-butadiene 1,2,3-butatriene + Geometric Isomers Geometric isomers are present in substances that have double bonds because of the nature of the double bond. Single bonds allow free rotation around the bond. However, double bonds have more of a rigid/locked formation so rotation is not free…it is restricted. This restricted nature comes from the fact that not only are sigma bonds involved, but p-orbital overlap occurs above and below the sigma bond. These isomers are called either cisor trans- depending on the location of groups branching off the double bond carbons. These isomers are not superimposable, meaning that they have different arrangements in space when placed on top of one another. They also have different physical properties as well. + Cis- vs. Trans The green atoms are Cl. You can see how the groups are called cis- if they are on the same, or adjacent side of the molecule. They are called trans- if they are on opposite sides of the molecule. This is works for other groups/branches that come off of double bonds. + Practice Draw these structures: Identify these as cis/trans: 1) trans-3-heptene 1) 2) cis-5-chloro-2-hexene Orient the cis/trans designation of the longest carbon chain that surrounds(includes) the double bond. 2) + Cycloalkenes These are cyclic compounds that contain a C-C double bond in the ring. The carbons of the double bond are numbered 1 and 2, so no numbers are needed. Most common cycloalkenes: Cyclopentene Other branches are numbered with the lowest number possible, after numbering the carbons of the double bond first. Cyclohexene Practice: + Preparation of Alkenes Most are prepared from saturated hydrocarbons. Cracking- process in which saturated hydrocarbons are heated to very high temps. In the presence of a catalyst (usually silica-alumina), the hydrocarbon breaks into smaller molecules. Some eliminate hydrogen to form alkenes. Results in mixtures of hydrocarbons… Dehydration of Alcohols- Water is removed from an alcohol molecule (characterized by an OH group) in the presence of concentrated sulfuric acid. See below… + Physical Properties of Alkenes Boiling points are slightly lower than corresponding alkanes. Melting points of isomers are wildly different than boiling points. Chains with 4 or fewer carbons are gases at room temp. This is because the structure of the isomer would affect the crystal structure. 5-17 C chain length are liquids. Higher than 17, usually a solid. Densities are less than water. + Chemical Reactions involving Alkenes Alkenes are more reactive than alkanes, mostly because there are fewer than 4 bonds to C in alkenes and alkynes. Addition Reaction- most common Hydrogen, Halogens, Hydrogen halides, sulfuric acid and water are some substances that can be added to alkenes. Can be converted to saturated molecules (alkanes) by addition with hydrogen.