Chapter 4: Alkanes and Cyclohexenes
Chapter 4: Alkanes and Cyclohexenes CHMY 321-001
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This 7 page Class Notes was uploaded by Rebeka Jones on Sunday October 16, 2016. The Class Notes belongs to CHMY 321-001 at Montana State University taught by Holmgren, Steven in Fall 2016. Since its upload, it has received 5 views.
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Date Created: 10/16/16
Chapter 4: Alkanes and Cyclohexenes Reminder hydrocarbons are just C and H bonds. Hydrocarbons that lack pi bonds are called saturated hydrocarbons or alkanes. Nomenclature – the system for naming chemical compounds Names provided by IUPAC rules are called systematic names. The first step in naming an alkane is to identify the longest chain called the parent chain. If there is two chains of equal length, then choose the chain with the greater number of substituents. Substituents – groups connected to the parent chain. The term “cyclo” is used to indicate the presence of a ring in the structure of an alkene. Once the parent has been identified list all substituents. When naming branched substituents number each carbon going away from the parent chain. Place the numbers on the longest chain in the substituent. Treat the complex substituent as if it is a mini-parent with its own substituents when naming it place it in parentheses. - An alkyl being 3 carbons can only be branched in one way and it is called an isopropyl group. - Alkyl groups with 4 carbons can be branched in 3 different ways. - Alkyl groups with 5 carbons can be branched in many different ways. In order to assemble the systematic name number, the carbons of the parent chain. These numbers are called locants. Rules - If one substituent is present it should be assigned the lowest possible number. - When multiple substituents are present, assign numbers so the first substituent receives the lowest number. - If there is a tie the second locant should be as low as possible. - If there is still a tie, then the lowest number should be assigned alphabetically. *when a substituent appears more than once in a compound a prefix is used to identify how many times *hyphen separate numbers from letters and commas separate numbers from numbers. *once all substituents have been identified and assigned arran ge alphabetically (ignore all prefixes, sec, and tert) In summary - Identify parent chain - Identify name and substituents - Number parent chain and assign locants - Arrange the substituents alphabetically 2 Compound that contain two fused rings are called bicyclic. Bridge head – the two carbon atoms where the rings are fused together. There are different paths connecting two bridge heads. For each compound, count the number of carbon atoms, excluding the bridge heads. These numbers ordered form the longest to smallest are pla ced in the middle of parent surrounded by brackets. Ex: Bicyclo[2,2,1] heptane These numbers provide information on specific differences between compounds. To number the parent, start at the bridge head that gives the substituent the lowest locant following numbering systems from the longest path to the shortest. Regardless of the position of substituent, the parent must be numbered beginning with the longest path first. But must start at the bridge head giving the substituent the lowest possible number. For an alkane, the number of possible constitutional isomers increases with increasing molecular size. - Avoid draw the same isomer twice o Use IUPAC names to know for sure In order to compare stabilities of alkanes we compare the heat released when combusted. DH = Change in enthalpy -DH° = heat of combustion 3 Branched alkanes are lower in energy (more stable) than straight -chain alkanes. Processes that are employed to increase gasoline yield. - Cracking: process where C-C bonds of larger alkanes are broken producing smaller alkanes suitable for use as gasoline. o Done by high temperatures o Generally, leads to straight chain alkanes - Reforming: process of many reactions that lead to converting straight chain alkanes into branched hydrocarbons and aromatic compounds o More desirable Conformations = 3D shapes Newman projections – drawing specifically designed to show the conformation of molecules - One carbon directly in front of the other - Point in the center of drawing = front carbon - Circle represents back carbon Drawing Newman projections - Identify the three groups connected to front carbon atom - Identify the three group connected to the back carbon atom - Draw Newman projection Torsion angle = separated by 60 degrees Staggered conformations – lowest energies Eclipsed conformations – highest energies Torsional strain – difference in energy between staggered and eclipsed 4 Anti-conformation = 180 degrees Gauche interactions – type of steric interactions Angle strain – the increase in energy associated with a bond angle that has deviated from the preferred angle of 109.5 degrees Bonds of a larger cyclohexane can position themselves 3 -dimensionally so to achieve a conformation that minimizes the total energy of the compound Cyclopropane has two main factors contributing to its high energy: angle strain and torsional strain - Prone to ring opening reactions Clyclobutane has less angle strain but more torsional strain Cyclopentane has less totally strain but still exhibits some strain. Cyclohexane can adopt many conformations some examples are the chair and the boat. - Bond angles close to 109.5 = little angle strain - Significant different is caused by torsional strain o chair has none o boat has two sources – many H’s are eclipsed and H’s on either side of the ring experience flag pole interactions. § Can alleviate by twisting to form a twist boat. Each carbon atom in a cyclohexane ring can bear two substituents - Axial position: parallel to a vertical axis passing through the center of the ring 5 - Equatorial position: approximately along the equator of the ring For a ring containing only one substituent two possible chair conformation can be drawn The substituent can be in an axial position or in a equatorial position. These two possibilities represent two different conformations that are in equilibrium to each other. The term “ring flip” is used to describe the conversion of one chair conformation into the other done by rotating all C-C bonds. When two chair conformations are in equilibrium, the lower energy conformation will be favored. When the substituent is in axial position, there are strain interactions with the other axial H’s on the same side of the ring. The substituent’s electron cloud is trying to occupy the same region of space causing steric interactions. These are called 1,3-diaxial interactions. The presence of 1,3-diaxial interactions causes the chair conformations to be higher in energy when the substituent is in the axial position. When the substituent is in an equatorial position these 1,3-diaxial interactions are absent. The two chair conformations will generally favor the conformation with the equatorial substituent. When drawing chair conformations of compound with two or more substituents we must also consider the 3D orientation of each substituent . Wedge = up 6 Dash= down *ring flip does not change conformation when you have a contradicting substituents you have to decide which group has lower energy in equatorial. Cis- two groups that are on the same side of the ring Trans – two groups that are on opposite sides of the ring Stereoisomers – different compounds with different physical properties, and they cannot be interconverted via conformational change. 7
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