1 Announcements & Agenda (03/05/07) Pick up Movie Quiz Exam next Wednesday! Covers Ch 7, 8, Covers Ch 7, 8, Will post a practice exam by Wed Will post a practice exam by Wed Grade sheets will be given on Wed (sorry  ) Grade sheets will be given on Wed (sorry  ) Come visit Bob!!! Come visit Bob!!!Today Cis-trans isomers (11.2) Cis-trans isomers (11.2) Reactivity of unsaturated compounds (11.3) Reactivity of unsaturated compounds (11.3) Aromatic Compounds (11.5) Aromatic Compounds (11.5)

2 Last Time: Functional Groups (Know these!!!) These are some of the most important biological functional groups!

3 have fewer H atoms attached to the C chain than alkaneshave fewer H atoms attached to the C chain than alkanes are alkenes with double bondsare alkenes with double bonds If one C=C bond, C n H 2n If one C=C bond, C n H 2n Monounsaturated = 1 D.B. Monounsaturated = 1 D.B. are alkynes with triple bonds If one C ≡ C bond, C n H 2n-2 If one C ≡ C bond, C n H 2n-2 Last Time: Saturated vs. Unsaturated Hydrocarbons Unsaturated hydrocarbons Naming: Know “ene” means alkene group present Know “yne” means alkyne group present

4 Last Time: Unsaturated Fatty Acids Fatty acids in vegetable oils are omega-6 acids (the first double bond occurs at carbon 6 counting from the methyl group) Fatty acids in vegetable oils are omega-6 acids (the first double bond occurs at carbon 6 counting from the methyl group) A common omega-6 acid is linoleic acid A common omega-6 acid is linoleic acid CH 3 CH 2 CH 2 CH 2 CH 2 CH=CHCH 2 CH=CH(CH 2 ) 7 COOH 6 linoleic acid, a fatty acid an alkene group a carboxylic acid group

5 Last Time: Unsaturated compounds are oxidized relative to saturated compounds Oxidation of organic compounds means: Replacement of hydrogen atom with a more electronegative atom (oxygen or nitrogen) Replacement of hydrogen atom with a more electronegative atom (oxygen or nitrogen)OR Loss of H atoms & replacement with a double bond Loss of H atoms & replacement with a double bond

6 Bond Angles in Alkenes & Alkynes According to VSEPR theory: the 3 groups bonded to C atoms in a double bond are at 120° angles.the 3 groups bonded to C atoms in a double bond are at 120° angles. alkenes are flat because the atoms in a double bond all lie in the same plane.alkenes are flat because the atoms in a double bond all lie in the same plane. the 2 groups bonded to each carbon in a triple bond are at 180° angles.the 2 groups bonded to each carbon in a triple bond are at 180° angles.

7 How do you make a double bond? The 1st bond is made by “orbitals” overlapping between atoms, and is called a “sigma” (σ) bond. The 2nd bond is made by sideways p orbitals (not directly between the atoms) and is called a “pi” (π) bond. This is why a single bond can rotate. But the π bond part of a double bond can’t rotate – you’d have to break it.

8 Steroisomers: Cis-Trans Isomers In an alkene, the double bond is rigid, i.e. NO ROTATION around the double bond!is rigid, i.e. NO ROTATION around the double bond! holds attached groups in fixed positions.holds attached groups in fixed positions. makes cis/trans isomers possible.makes cis/trans isomers possible. Important in nature – such as in pheromonesImportant in nature – such as in pheromones CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH = CH CH = CH CH = CH CH = CH cis trans CH 3 cis trans CH 3 (similar groups on same side) (similar groups on opp. sides)

9 Cis-Trans Isomerism Alkenes cannot have cis-trans isomers if a C atom in the double bond is attached to identical groups.Alkenes cannot have cis-trans isomers if a C atom in the double bond is attached to identical groups. Identical Identical 2-bromopropene 1,1-dibromoethene 2-bromopropene 1,1-dibromoethene (not cis or trans) (not cis or trans) (not cis or trans) (not cis or trans) H H H Br

10 Cis-Trans Isomerization of Retinal From Lehninger Principles of Biochemistry 3rd ed. Present in Rhodopsin

11 These guys are… 1.The exact same compound 2.Stereoisomers (cis/trans) 3.Constitutional (structural) isomers 4.Different compounds that aren’t isomers

12 These guys are… 1.The exact same compound 2.Stereoisomers (cis/trans) 3.Constitutional (structural) isomers 4.Different compounds that aren’t isomers

13 These guys are… 1.The exact same compound 2.Stereoisomers (cis/trans) 3.Constitutional (structural) isomers 4.Different compounds that aren’t isomers 12345

14 Reactions of Alkenes 11.3 Addition Reactions

15 Addition Reactions The double/triple bond can be brokenThe double/triple bond can be broken alkenes & alkynes are very reactive. alkenes & alkynes are very reactive. In addition reactions, reactants are added to the carbon atoms in the double or triple bond.In addition reactions, reactants are added to the carbon atoms in the double or triple bond.

16 H 2 adds to the C atoms of a double/triple bond.H 2 adds to the C atoms of a double/triple bond. A catalyst (e.g. Pt or Ni) is used to speed up the reaction.A catalyst (e.g. Pt or Ni) is used to speed up the reaction. Hydrogenation

17 Adding H 2 to double bonds in vegetable oils produces: compounds with higher melting points.compounds with higher melting points. solids at room temperature such as margarine,solids at room temperature such as margarine, soft margarine, soft margarine, and shortening. and shortening. Hydrogenation of Oils

18 Trans Fats In vegetable oils, the unsaturated fats usually contain cis double bonds. During hydrogenation, some cis double bonds are converted to trans double bonds (more stable), which causes a change in the fatty acid structure.During hydrogenation, some cis double bonds are converted to trans double bonds (more stable), which causes a change in the fatty acid structure. If a label states “partially” or “fully hydrogenated”, the fats contain trans fatty acids.If a label states “partially” or “fully hydrogenated”, the fats contain trans fatty acids.

19 Trans Fats: So What? In the US, estimate 2-4% of our total Calories is in the form of trans fatty acid. trans fatty acids behave like saturated fatty acids in the body. Several studies reported that trans fatty acids raise LDL-cholesterol. Some studies also report that trans fatty acid lower HDL-cholesterol

20 Hydration: Another Addition Reaction an acid H + catalyst is required.an acid H + catalyst is required. water (HOH) adds to a double bond.water (HOH) adds to a double bond. an H atom bonds to one C in the double bond.an H atom bonds to one C in the double bond. an OH bonds to the other C.an OH bonds to the other C. H OH H OH H + │ │ H + │ │ CH 3 ─CH=CH─CH 3 + H─OH CH 3 ─CH─CH─CH 3

21 Hydration When hydration occurs with a double bond that has an unequal number of H atoms, the H atom bonds to the C in the double bond with the more H.the H atom bonds to the C in the double bond with the more H. the OH bonds to the C in the double bond with the fewer H atoms.the OH bonds to the C in the double bond with the fewer H atoms. OH H OH H H + │ │ H + │ │ CH 3 ─CH=CH 2 + H─OH CH 3 ─CH─CH 2

22 Learning Check Write the addition product for each of the following: H + H + 1. CH 3 ─CH 2 ─CH=CH─CH 2 ─CH 3 + HOH CH 3 CH 3 │ H + │ H + 2. CH 3 ─C=CH─CH 2 ─CH 3 + HOH 3. CH 3 ─CH=CH─CH 3 + H HOH Pt H+

23 Solution H OH H OH │ │ │ │ 1. CH 3 ─CH 2 ─CH─CH─CH 2 ─CH 3 CH 3 CH 3 │ 2. CH 3 ─C─CH─CH 2 ─CH 3 3. CH 3 ─CH 2 -CH 2 ─CH 3 │ │ │ │ OH H OH H OH OH4. H

24 One Last Thing about Hydration… Hydration is also reversible – the equilibrium amounts of alkene compared to alcohol depend on how much water is around. Lots of water – the reaction tends to use up as much water as it can, and most of the alkene forms alcohol. If water is removed, water can be pulled off from alcohol to form the alkene. (concentrated acid).

25 DNA damage by UV radiation: induces dimerization of DNA bases (particularly T-T) Spring Breakers: Why too much UV is bad… A Reaction of Double Bonds!

26 Conjugated bonds If you have alternating double & single bonds, you get “extended, conjugated” pi bonds. you get “extended, conjugated” pi bonds. what looks like a single bond in between two double bonds, is actually sort of a double bond itself. what looks like a single bond in between two double bonds, is actually sort of a double bond itself. multiple pairs of electrons can spread out all the way through the aligned parallel orbitals -> molecules like this! multiple pairs of electrons can spread out all the way through the aligned parallel orbitals -> molecules like this!

27 Why is blood red? Delocalization of course! It’s the HEME in hemoglobin!!! It’s the HEME in hemoglobin!!!

28 One Important Example: Benzene (Know This One!) a ring of 6 C & 6 H atoms.a ring of 6 C & 6 H atoms. a flat ring structure drawn with three double bonds.a flat ring structure drawn with three double bonds. represented by 2 structures because electrons are shared among all the C atoms.represented by 2 structures because electrons are shared among all the C atoms. Aromatic Compounds “Special Conjugation”

29 Benzene and Aromaticity Aromatics are compounds that have conjugated pi bonds around a ring. Electrons love this, and aromatics are very stable.

30 Occurrences in Nature & Health Vanillin Aspirin Ibuprofen Acetaminophen Aromatics can be bad too  Many are carcinogens! Can interact with DNA.

31 Aromatic rings can include N atoms, too. They can be 5 or 6 membered rings (and >), fused rings, etc. Adenine Thymine Phenylalanine Pyridine