Lecture and Chapter 7 Notes

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Thavarajah/ Woolley/ Nitz

CHM138H1 Jasmyn Lee Chapter 7: Alkenes: Structure and Reactivity 7.1 Industrial Properties and Use of Alkenes  Ethylene can be produced by “thermal cracking” at very high temperatures 7.2 Calculating the Degrees of Unsaturation of a Molecule  A degree of unsaturation refers to a double bond or a ring in a system o Each ring or double bond corresponds to a loss of 2 H’s  Alkenes are unsaturated compounds (contain at least one double bond)  Degrees of Unsaturation can be calculated from the Molecular Formula 1 CHM138H1 Jasmyn Lee  Steps 1. Look at number of carbons in the formula 2. Write out the formula of the Linear Alkane with same number of carbons 3. Calculate: [ ( # of H in alkane) – ( # of H in compound) ] / 2  What to do about –X, -O-, N o Halogens (-X) – replace each halogen with H and do the same calculation o Oxygen (-O-) – remove all oxygen’s from formula and do the same calculation o Nitrogen (N) – remove N and one H (for each nitrogen) and do same calculation 7.3 Naming Alkenes  Suffix: ene 1. Name the parent – find the longest carbon chain that contains the double bond ( both sp carbons) 2. Number the chain – start with the end closest to the double bond st 2 3. Indicate the position of the double bond by giving the number of the 1 sp carbon 4. Indicate substituents as for alkanes 5. For rings, one double bond is located between C1 and C2  Common names:  Name the parent hydrocarbon 2 CHM138H1 Jasmyn Lee  Number the carbons in chain so that double bond carbons have lowest possible numbers  Write the full name, number substituents according to o Position in chain o Alphabetically  Rings have a “cyclo” prefix 7.4 Cis-Trans Isomerism in Alkenes Cannot Rotate a Double Bond Without Breaking It  Rotation of π bond is prohibitive  This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond)  Creates possible alternative structures  These two compounds are identical; they are not cis-trans isomers  These two compounds are not identical; they are cis-trans isomers  Cis-Trans naming system works with disubstituted alkenes (compounds that have two substituents other than hydrogen on the double bond) 3 CHM138H1 Jasmyn Lee 7.5 Alkene Stereochemistry and the E,Z Designation Naming Tri and Tetra Substituted Double Bonds  Priority rules of Cahn, Ingold, and Prelog  Compare where higher priority groups are with respect to bond and designate as prefix  E – entgegen, opposite sides o E double bond o Higher ranked groups are on opposite sides  Z – zusammen, together on the same side o Z double bond o Higher ranked groups on the same side Naming  Note: Disubstituted alkenes can be named with cs/trans OR E/Z  (Z)-4-choro-1-fluoro-2,4-dimethyl-2-hexane 7.6 Stability of Substituted Alkenes  Cis-2-butene is less stable than trans-2-butene – due to steric strain  Alkene stabilities can be measured in a different ways o More stable alkene gives off less heat when hydrogenated – trans-butene generates ~5kJ/mol less heat than cis-butene 4 CHM138H1 Jasmyn Lee  More substituted alkenes tend to be more stable, due to 1. Hyperconjugation – a stabilizing interaction between a vacant C=C π antibonding orbital (associated with double bond) and a sp hybrid on the adjacent carbon (substituents) 2. Bond Strength – a bond between an sp carbon and an sp carbon is stronger than a bond between two sp 3 carbons 2 3 o More highly substituted alkenes have a higher ratio of sp -sp C bonds: 7.7 Electrophilic Addition Reactions of Alkenes Addition of HX to an Alkene Electrophilic Addition  X is usually Cl, Br, I or OH
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