Announcements & Agenda (03/05/07)

Announcements& Agenda(03/05/07) • Pick up Movie Quiz • Exam next Wednesday! • Covers Ch 7, 8, 10-12 • Will post a practice exam by Wed • Grade sheets will be given on Wed (sorry ) • Come visit Bob!!! • Today • Cis-trans isomers (11.2) • Reactivity of unsaturated compounds (11.3) • Aromatic Compounds (11.5)

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

Last Time: Saturated vs. Unsaturated Hydrocarbons Unsaturated hydrocarbons • have fewer H atoms attached to the C chain than alkanes • are alkenes with double bonds • If one C=C bond, CnH2n • Monounsaturated = 1 D.B. • are alkynes with triple bonds • If one C≡C bond, CnH2n-2 Naming: Know “ene” means alkene group present Know “yne” means alkyne group present

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) • A common omega-6 acid is linoleic acid CH3CH2CH2CH2CH2CH=CHCH2CH=CH(CH2)7COOH 6 linoleic acid, a fatty acid an alkene group a carboxylic acid group

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) OR • Loss of H atoms & replacement with a double bond

Bond Angles in Alkenes & Alkynes According to VSEPR theory: • 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. • the 2 groups bonded to each carbon in a triple bond are at 180° angles.

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.

Steroisomers: Cis-Trans Isomers In an alkene, the double bond • is rigid, i.e. NO ROTATION around the double bond! • holds attached groups in fixed positions. • makes cis/trans isomers possible. • Important in nature – such as in pheromones CH3 CH3 CH3 CH = CH CH = CH cistransCH3 (similar groups on same side) (similar groups on opp. sides)

Cis-Trans Isomerism • Alkenes cannot have cis-trans isomers if a C atom in the double bond is attached to identical groups. Identical Identical 2-bromopropene1,1-dibromoethene (not cis or trans) (not cis or trans) H H Br H

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

These guys are… • The exact same compound • Stereoisomers (cis/trans) • Constitutional (structural) isomers • Different compounds that aren’t isomers

Reactions of Alkenes 11.3 Addition Reactions

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

Hydrogenation • H2 adds to the C atoms of a double/triple bond. • A catalyst (e.g. Pt or Ni) is used to speed up the reaction.

Hydrogenation of Oils Adding H2 to double bonds in vegetable oils produces: • compounds with higher melting points. • solids at room temperature such as margarine, soft margarine, and shortening.

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. • If a label states “partially” or “fully hydrogenated”, the fats contain trans fatty acids.

0 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

Hydration: Another Addition Reaction • an acid H+ catalyst is required. • water (HOH) adds to a double bond. • an H atom bonds to one C in the double bond. • an OH bonds to the other C. HOH H+│ │ CH3─CH=CH─CH3 + H─OH CH3─CH─CH─CH3

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 OH bonds to the C in the double bond with the fewer H atoms. OHH H+│ │ CH3─CH=CH2 + H─OHCH3─CH─CH2

Learning Check Write the addition product for each of the following: H+ 1. CH3─CH2─CH=CH─CH2─CH3 + HOH CH3 │ H+ 2. CH3─C=CH─CH2─CH3 + HOH 3. CH3─CH=CH─CH3 + H2 4. + HOH Pt H+

Solution H OH │ │ 1. CH3─CH2─CH─CH─CH2─CH3 CH3 │ 2. CH3─C─CH─CH2─CH3 3. CH3─CH2-CH2─CH3 │ │ OH H OH 4. H

0 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).

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

0 Conjugated bonds • If you have alternating double & single 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. • multiple pairs of electrons can spread out all the way through the aligned parallel orbitals -> molecules like this!

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

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

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

Occurrences in Nature & Health VanillinAspirin IbuprofenAcetaminophen Aromatics can be bad too  Many are carcinogens! Can interact with DNA.

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

Announcements & Agenda (03/05/07)