Chem 150 Unit 4 - Chemical Properties I Chemical Reactions

1. Chem 150Unit 4 - Chemical Properties IChemical Reactions • The chemical properties of molecules describe processes that involve the making and breaking the stronger covalent bonds that hold molecules together. As a consequence, the compositions of the the molecules participating in a chemical process change. These processes are called chemical reactions and can be expressed using balanced chemical equations.

2. Introduction • Chemical reactions involve the rearrangement of the atoms within and between molecules that results in the formation of new molecules. • This process involves the making and breaking of covalent bonds. • An important concept in these processes is that all of the atoms present before a reaction are also present after the reaction • This a concept allows us to describe chemical reactions using chemical equations

3. Introduction • If you need to review how to create and balance chemical equation, take a look at Section 6.1 in Raymond. • We will focus on some reactions that are important in biological chemistry, including: • Oxidation/Reduction reactions • Reactions involving water • We will also look at the different forms of free energy that can be used to predict the directions and rates of chemical reactions.

4. Question • When you are driving along in your automobile, octane in the gasoline is reacting with oxygen from the air to produce carbon dioxide and water. Write a balanced chemical equation that can be used to describe this reaction 2C8H18 +25 O2 --> 16 CO2 + 18 H2O

5. Oxidation and Reduction • In Unit 1 we discussed some of the strategies that atoms use to obtain 8 valence electrons. • See Unit IElaboration - The Octet Rule • See Unit 1Elaboration - Compounds

6. Oxidation and Reduction • When metal atoms combine with non-metal atoms, they transfer electrons from the metal to the non-metal to form ionic compounds: • Sodium, Na (s), is a soft grey metal. • Chlorine, Cl2 (g), is toxic green gas. • Sodium chloride, NaCl (s), is a crystalline white solid comprising sodium ions, Na+, and chloride ions, Cl–.

7. Oxidation and Reduction • Reactions that involve the transfer of electrons from one atom to another are called oxidation/reduction reactions. • The atom losing the electrons is oxidized. • In the previous example, the sodium is oxidized: • The atom gaining the electrons is reduced. • In the previous example, the chlorine is reduced: • While the two processes can be separated, one cannot occur without the other.

8. Oxidation and Reduction • Reactions that involve the transfer of electrons from one atom to another are called oxidation/reduction reactions. • The reactant that takes away the electrons is the oxidizing agent. • In the previous example, the chlorine is the oxidizing agent. • The chlorine took the electrons away from the sodium. • The reactant that donates the electrons is the reducing agent. • In the previous example, the sodium is the reducing agent. • The sodium gave the electrons to the chlorine.

9. Oxidation and Reduction • In oxidation and reduction, metals can also transfer electrons between themselves: • Copper, Cu (s), is a reddish metal. • Silver(I) nitrate, AgNO3 (aq), a colorless aqueous solution containing silver(I) ions, Ag+ ions and nitrate ions, NO3-. • copper(II) nitrate, Cu(NO3)2 (aq), a green aqueous solution containing copper(II) ions, Cu2+, ions and nitrate ions, NO3-. • Silver, Ag (s), a silvery metal

10. Oxidation and Reduction • Oxidation and Reduction • The atom losing the electrons is oxidized. • In the previous example, the copper is oxidized: • The atom gaining the electrons is reduced. • In the previous example, the silver(I) ion is reduced:

11. Questions (Clickers) • Zinc reacts with copper(II) sulfate according to the equation: • Is the zinc being • Oxidized? • Reduced?

12. Questions (Clickers) • Zinc reacts with copper(II) sulfate according to the equation: • Is the copper(II) ion being • Oxidized? • Reduced?

13. Questions (Clickers) • Zinc reacts with copper(II) sulfate according to the equation: • Which reactant is the oxidizing agent? • Zinc • Copper(II) ion

14. Questions (Clickers) • Zinc reacts with copper(II) sulfate according to the equation: • Which reactant is the reducing agent? • Zinc • Copper(II) ion

15. Oxidation and Reduction • Oxidation/reduction reactions can also occur when no ions or metals are involved. • This can occur when molecular compounds composed of nonmetals react with one another to form other molecular compounds, • And: • Polar covalent bonds are produced or eliminated • Or • Double or triple bonds are produced or eliminated • See Unit 1Elaboration - Polarity

16. Oxidation and Reduction • The combustion of an organic molecule to produce carbon dioxide and water is an example: • The products of this reaction contain polar covalent bonds in which the electrons are being drawn away from • The carbon atom in CO2 • The hydrogen atoms in H2O • The carbons and the hydrogens are being oxidized. • The oxygen is being reduced.

17. Oxidation Reduction An atom loses electrons An atom gains electrons An atom gains a bond to oxygen An atom loses a bond to oxygen An atom loses a bond to hydrogen An atom gains a bond to hydrogen Oxidation and Reduction • An easier way to assess whether a reaction is an oxidation/reduction reaction or not is to look for the following:

18. Oxidation and Reduction • Applying these rules to the combustion of methane: • The carbon is being oxidized because it gains bonds to oxygen. • The carbon is also being oxidized because it is losing bonds to hydrogen. • The hydrogens are being oxidized because they gain bonds to oxygen.

19. Oxidation and Reduction Rules for Assigning Oxidation Numbers * The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element. Thus, the atoms in O2, O3, P4, S8, and aluminum metal all have an oxidation number of 0. * The oxidation number of monatomic ions is equal to the charge on the ion. The oxidation number of sodium in the Na+ ion is +1, for example, and the oxidation number of chlorine in the Cl- ion is -1. * The oxidation number of hydrogen is +1 when it is combined with a nonmetal. Hydrogen is therefore in the +1 oxidation state in CH4, NH3, H2O, and HCl. * The oxidation number of hydrogen is -1 when it is combined with a metal. Hydrogen is therefore in the -1 oxidation state in LiH, NaH, CaH2, and LiAlH4. * The metals in Group IA form compounds (such as Li3N and Na2S) in which the metal atom is in the +1 oxidation state. * The elements in Group IIA form compounds (such as Mg3N2 and CaCO3) in which the metal atom is in the +2 oxidation state. * Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O2, O3, H2O2, and the O22- ion. * The nonmetals in Group VIIA often form compounds (such as AlF3, HCl, and ZnBr2) in which the nonmetal is in the -1 oxidation state. * The sum of the oxidation numbers of the atoms in a molecule is equal to the charge on the molecule. * The most electronegative element in a compound has a negative oxidation number. http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch19/oxred_1.php#assign

20. Oxidation Reduction An atom loses electrons An atom gains electrons An atom gains a bond to oxygen An atom loses a bond to oxygen An atom loses a bond to hydrogen An atom gains a bond to hydrogen Oxidation and Reduction • An easier way to assess whether a reaction is an oxidation/reduction reaction or not is to look for the following:

21. Oxidation and Reduction • Hydrogenation • Another type of oxidation/reduction reaction is the hydrogenation reaction: • In this example, an alkene is reduced to an alkane. • This is considered reduction, because the hydrogen is bringing in additional electrons to the molecule. • The alkane that is produced in this reaction is considered “saturated” because it can no longer absorb any more hydrogen atoms. unsaturated saturated

22. Oxidation and Reduction • Often chemist use a shorthand method of writing equations like these: • The equation shown on the previous slide can be written as follows: • One of the reactants, H2, is placed above the reaction arrow • Technically, this equation is no longer balanced • The shorthand method of writing a chemical equation is used to emphasize what happens to a key component of the reaction • In this case it is the alkene.

23. Oxidation and Reduction • Saturated vs Unsaturated Fats

24. Oxidation and Reduction • Saturated vs Unsaturated Fats

25. Oxidation and Reduction • Saturated vs Unsaturated Fats

26. Oxidation and Reduction • Saturated vs Unsaturated Fats

27. Oxidation and Reduction • Saturated vs Unsaturated Fats Fat (Triacylglyceride)

28. Oxidation and Reduction • Dehydrogenation • Oxidation/reduction also occurs when hydrogens are taken away from a molecule. This is called dehydrogenation. • The oxidation of succinic acid to fumaric acid: • This reaction takes place in the Citric Acid Cycle. • We will discuss the Citric Acid Cycle in Unit 12. • The FAD is an abbreviation for a large organic molecule called Flavin Adenine Dinucleotide.

29. Oxidation and Reduction • The reaction equation on the previous slide also illustrates another shorthand method of writing equations, which used multiple reaction arrows. • The longhand form of this reaction equation is

30. Oxidation and Reduction • Dehydration example • The oxidation of ethanol to form acetaldehyde: • This reaction occurs in liver after consuming alcohol. • The NAD+ is an abbreviation for a large organic molecule named Nicotinamide Adenine Dinucleotide.

31. Questions (Clickers) • In the following reaction, what is the ON# of carbon #1 vs. carbon #2? • 0 and -1 • 0 and +1 • -1 and +1 • None, this is not a redox reaction +1 +1 +1 +1 #2 #1 -3 -2 +1 +1 -2 +1 +1 +1 +1

32. Questions (Clickers) • In the following reaction, what role is played by the NAD+ • reducing agent • oxidizing agent • neither

33. Reactions Involving Water • While the major role for water in biology is a physical one as the primary solvent in living cell, it also plays a chemical role as a reactant or product in some chemical reactions. • Reactions involving water as a reactant or product • Acid-catalzyed hydrolysis • Base-catalyzed hydrolysis • Hydration • Dehydration

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35. Nobel prize in chemistry-2008 • Chalfie, Chimomura and Tsien-Green Fluorescent Protein-GFP A whole bunch of new fluorescent proteins-Roger Tsien Brainbows!

36. Reactions Involving Water • Hydrolysis • In the hydrolysis reaction, water (hydro) is used to split (lyse) another molelcule. • In this case, water is being used to split an ester into a carboxylic acid plus and alcohol.

37. Reactions Involving Water • Hydrolysis example • The hydrolysis of the ester bond in the neurotransmitter acetylcholine. • Again, shorthand notation is being used: • The H2O reactant is placed above the reaction arrow, • The H+ below the arrow indicates an acid catalyst is used.

38. Reactions Involving Water • Hydrolysis • Hydrolysis can also be catalyzed using a base (OH-):. • Because one of the products of the hydrolysis is a carboxylic acid, in base catalyzed hydrolysis the base undergoes a second acid/base reaction with the carboxylic acid to produce a carboxylate ion. • The base catalyzed hydrolysis of esters is also called saponification • We will be discussing acids and bases in Unit 6

39. Reactions With Water • Hydrolysis example: • The base catalyzed hydrolysis of fats produces soap and glycerol Fat

40. Reactions With Water • Hydrolysis example: • The base catalyzed hydrolysis of fats produces soap and glycerol Soap Glycerol

41. Reactions Involving Water • Hydration • In the hydration reaction water is also split, but instead of being used to split another molecule, it is added to another molecule to produce a single product. • The water it is added to either an alkene or alkyne: • The hydration of an alkene produces an alcohol.

42. Reactions Involving Water • Hydration • This can also be written in shorthand as: • The H+ below the reaction arrow is used to indicate that this is an acid-catalyzed reaction. • The shorthand is used to emphasize what happens to the key reactant.

43. Reactions Involving Water • Hydration example • On an earlier slide a reaction from the Citric Acid Cycle was shown, which involved the dehydrogenation of succinic acid to produce fumaric acid. • The sequent reaction in the Citric Acid Cycle is an example of a hydration reaction:

44. Reactions Involving Water • Dehydration • In the dehydration reaction is the reverse of the hydration reaction. • The water it is removed from an alcohol: • The dehydration of an alcohol produces an alkene.

45. Reactions Involving Water • Dehydration example • The Citric Acid Cycle also provides a good example of a dehydration reaction. • A dehydration reaction followed by a hydration reaction is used to move a hydroxyl group from one carbon to an adjacent carbon in citric acid:

46. Free Energy and Reaction Rates • In Unit 3 we discussed how changes in the free energy can be used to predict whether a process is spontaneous (favorable) or nonspontaneous (not favorable) ΔG < 0 spontaneous ΔG > 0 nonspontaneous

47. Free Energy (G) Free Energy (G) Progress ofreaction Progress ofreaction Free Energy and Reaction Rates • The same principles can be applied to chemical reactions to predict whether they are favorable or not: Α → B Α → B ΔG > 0 nonspontaneous ΔG < 0 spontaneous A Β Β A

48. Free Energy and Reaction Rates • Just because a reaction is spontaneous, does not mean that it will occur at an observable rate. • For example, diamond and graphite are two different forms of pure carbon. The reaction that converts diamond to graphite is actually a favorable one • This does not make diamonds a bad investment for fear that they will turn into pencil lead. • Why? Diamond → Graphite ΔG < 0 spontaneous Diamond Free Energy (G) Graphite Progress ofreaction

49. Free Energy (G) Progress ofreaction Free Energy and Reaction Rates • There is is a hill that for most reactions the reactants must climb and go over to before they can go on to become product. Α → B A Β

50. Free Energy (G) Progress ofreaction Free Energy and Reaction Rates • The height of this hill is called the activation energy, Eact. • The activation energy has no effect on the overall change in the free energy for the reaction. Α → B Eact > 0 A ΔG < 0 spontaneous Β