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5 Organic Reactions to discuss….

5 Organic Reactions to discuss…. Addition Reactions Oxidation/Reduction Reactions Substitution Reactions Esterification Reactions Polymerization Reactions There are thousands of other organic reactions we will not cover!. 1. Addition Reactions.

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5 Organic Reactions to discuss….

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  1. 5 Organic Reactions to discuss…. • Addition Reactions • Oxidation/Reduction Reactions • Substitution Reactions • Esterification Reactions • Polymerization Reactions • There are thousands of other organic reactions we will not cover!

  2. 1. Addition Reactions • Must involve a double or triple bond – an alkene or alkyne! • As the name suggests, it involves adding a compound to the double bond • Sometimes a catalyst is used to speed up the reaction – this will be written over the arrow!

  3. What is the mechanism of these reactions? • A mechanism visually shows the steps that the molecules follow in order to complete the reaction… • Let’s look at a sample addition reaction…..

  4. This reaction is what we call stepwise – it occurs in steps! • H-Cl breaks into H+1 and Cl-1 in solution • It is an ionic compound with hydrogen acting as the metal • The H+1 attacks the double bond… • Why…..?

  5. The double bond is a great source of electrons! • Opposites attract! • This causes the electrons in the double bond to drop out and bond with the hydrogen! • This leaves the other C positive – why….? 1

  6. This carbon is missing a bond! • The formal charge on the carbon on the right is a +1 • This highly unstable atom is known as a carbocation • This atom will react immediately • Anything negative will attack it – in this case, the chlorine! • The new electrons for the bond come from the chlorine atom 1

  7. All addition reactions occur this way… • Something positive (called the electrophile) attacks the double bond…. • A carbocation is formed… • Something negative attacks the carbocation… • The positive atom or molecule is called the electrophile because it is electron loving, or seeking! Opposites attract!

  8. We can add a number of substances to double or triple bonds… • Halogens – Cl2, Br2, I2, F2 can be added with a uv light catalyst… • The mechanism follows the same pattern…. • attack of the double bond by something positive • Formation of a carbocation • Attack of the carbocation by something negative • Uv light acts as a catalyst to form two chlorine ions – one positive, one negative

  9. Eventually, both halogens add to the double bond… • This serves to increase the boiling point of the compound…. • Why…? • This is useful for solidifying hydrocarbons… • PVC is made this way…

  10. Addition of bromine….

  11. Alcohols can be made from water and an alkene or alkyne!

  12. Water can be forced to break into H+1 and OH-1 by the double bond…. • The H+1 attacks first… • The carbocation forms, and the OH-1 attacks the carbocation… • The overall effect is to create an alcohol!

  13. Catalytic Hydrogenation… • The addition of hydrogen using a catalyst.. • This breaks the double bond, adding two hydrogens… • This has the effect of raising the boiling point.. • This is how margarine and other fats are created from vegetable oil…

  14. The platinum atom holds the H2 into place • The hydrogens then split and attack the double bond in a similar manner as the other addition reactions • Oils and fats have many double and triple bonds • Some of these are broken, which is called partially hydrogenated • This has the effect of raising the boiling point slightly, changing the substance from a liquid to a thicker liquid or even a solid • The physical state is based on the amount of hydrogenation +

  15. Partial hydrogenation involves breaking some of these double or triple bonds • This changes the texture of the fat, and creates trans-fatty acids • These fats are unhealthy!

  16. Alkynes can undergo addition… Pt Pt In summary – an addition reaction eliminates double or triple bonds, and adds compounds or elements!

  17. Does it matter what carbon the positive atom attacks…? • The molecule in all previous examples, ethene, is symmetrical! • It doesn’t matter, because both carbons are identical! • A Russian scientist named Markovnikov found that with other alkenes and alkynes that are not symmetrical, it does matter!

  18. An addition reaction will occur… • Will the H+1 attack carbon 1 or carbon 2? • It matters… • Markovnikov’s Rule states that the carbocation will form on the more substituted carbon… • What does that mean…? • Let’s look at the two possible carbocations that can form!

  19. Which will form….? • The one that can be most stabilized will form! • Carbocations are unstable and must be stabilized by…. • Electrons! • Nearby C-H bonds that have electrons in them will flow over to stabilize the carbocation.. • This is called hyperconjugation!

  20. More hyperconjugation for stability… Less hyperconjugation for stability…

  21. The carbocation (positive carbon) will form on the atom that has more attached carbons, and therefore more hyperconjugation for stability…This is called Markovnikov’s Rule…. The negative ion then attacks the carbocation… minor

  22. How about this reaction….? Would you expect this….Why….?

  23. This forms instead! Why…..? The entire molecule rearranges to form a more stable carbocation! This is called rearrangement of the carbocation….

  24. The chlorine then attaches at carbon 2, not carbon 3!

  25. 2. Oxidation/Reduction Reactions • Oxidation, by definition, can mean three things: • Gaining oxygen • Losing hydrogen (dehydrogenation) and adding double or triple bonds • Losing electrons • Reduction, by definition, can mean three things: • Losing oxygen • Gaining hydrogen (hydrogenation – as in the addition reaction mentioned before!) and removing double or triple bonds • Gaining electrons

  26. Yes, this is an addition reaction – it is also a reduction reaction, because it involves adding hydrogens, and removing double and triple bonds….hydrogenation… Yes, this is the reverse of an addition reaction – it is an oxidation reaction, because it involves removing hydrogens, and adding double and triple bonds…dehydrogenation…. Pt Pt Notice the chemical written over the arrow – this is known as a catalyst…

  27. Oxidation reactions require a catalyst…. • Look for common catalysts in oxidation reactions: • K2Cr2O7 • H2SO4 • KMnO4 • Hydrocarbons follow a predictable pattern when oxidized… • An alcohol is produced – then an aldehyde – then a carboxylic acid – and eventually water and carbon dioxide

  28. Substitution reactions…. • A substitution reaction involves one molecule or atom substituting another in a hydrocarbon! • It has the general form: • Nu-1 is called a nucleophile…. It means nucleus loving, or seeking! They like positive charge… because they are negative!

  29. Carbon atoms have a tetrahedral shape around them… • This is crucial in determining the type of substitution reaction you will have… • Two types: • SN1 substitution • SN2 substitution • Let’s look at SN2 substitution first…

  30. What motivates the leaving group to actually leave….? • The incoming nucleophile! • The carbon, if it is going to bond to something new, has to eject something… • That something is called the leaving group! • What type of atoms would leave? • Ones that are highly electronegative, and would want to leave, pulling electrons off with it! • In this case, the chlorine! • Why is the nucleophile attacking from the back..? • The chlorine is in the way in the front! • The nucleophile can’t attack from the front! SN2 substitution….

  31. The leaving group takes the electrons with it… • The incoming group repels the electrons that are being shared with the hydrogens on the carbon – • Like charges repel! • This causes the atoms to reverse direction, or invert shape! • This is called inversion of configuration…. • This whole process happens all at once, because the only thing that would promote the Cl leaving would be something coming in from the other side to bond with the carbon! • This is called a concerted reaction – meaning, all at once!

  32. This reaction also occurs with backside attack, meaning that the nucleophile attacks from the backside of the molecule! • It is called SN2 because two molecules must collide to promote the reaction! • In summary, SN2 reactions occur: • Backside attack • Concerted • Inversion of configuration

  33. SN1 substitution… • This occurs when backside attack is not possible… • Why would backside attack not be possible…? • Too many carbons in the way! • This only occurs when you have a tertiarycarbon – a carbon that is attached to three other carbons! ONLY THREE CARBONS CAN EFFECTIVELY BLOCK BACKSIDE ATTACK – BACKSIDE ATTACK CAN OCCUR WITH ONE OR TWO CARBONS ATTACHED! • Backside attack is still possible with a secondary carbon – meaning, a carbon with two other carbon groups attached to it! Tertiary carbon Not possible!

  34. SN1 substitution… • So could frontside attack take place…? • NO! • There is a chlorine in the front in the way! • So we have to remove the chlorine… Must use something to remove the chlorine….

  35. SN1 substitution… • Normally, a solvent is used to remove the chlorine… • Ag+1 works well to pull off the chlorine… • The molecule now has a trigonal planar geometry around it, allowing the OH-1to attack from either side…! • However, a carbocation forms….! • But aren’t carbocations UNSTABLE…?

  36. There are nearby carbon-hydrogen bonds to stabilize the carbon…! • This is why the carbocation can form… • What would have happened if we had a primary (meaning no or one carbon attached) carbon? • There is no stabilization! • The molecule would go back to square one… • No reaction…. No stabilization!

  37. This is why primary carbons won’t occur frontside attack… • The solvent would pull off the leaving group, but there wouldn’t be enough nearby carbons to stabilize the carbocation… • Only secondary (2 carbons attached to the carbocation) or tertiary carbons can occur SN1…. • And you HAVE TO HAVE A SOLVENT TO PULL OF THE LEAVING GROUP!

  38. Both products are formed in an SN1 substitution… • Both frontside and backside attack can take place… • This can either invert the shape (if backside occurs) or retain the shape (if frontside occurs) • The reaction occurs in steps, or stepwise,where SN2 occurs all at once, or concerted • Both products are produced 50-50!

  39. This reaction is called SN1 because the speed of the reaction is dependent on only one step!

  40. SN1 versus SN2 substitution… http://www.colby.edu/chemistry/OChem/DEMOS/Substitution.html SN1 versus SN2 substitution… http://www.colby.edu/chemistry/OChem/DEMOS/Substitution.html • SN1 Substitution: • Must have a solvent to pull off leaving group • Must have a secondary or tertiary carbon to stabilize the carbocation • Can occur frontside or backside attack • Can retain or invert configuration • Occurs stepwise • SN2 Substitution: • Occurs with primary or secondary carbons • Only occurs backside attack • Always inverts configuration • Always occurs concertedly

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