Introduction to Organic and Biochemistry (CHE 124) Reading Assignment General, Organic, and Biological Chemistry: An Integrated Approach 3 rd. Ed. Ramond Chapter 5: Reactions Work Problems Chapter 5: 4, 6, 8, 10, 12, 14, 16, 66, 68, 70, 72, 74, 76, 78, 80,

Chemical Reactions Chemical Reactions – a chemical change in which the covalent or ionic bonds that hold elements or compounds together are broken and new bonds are formed. Chemical reactions are represented by chemical equations. C 6 H O 2 → CO 2 + H 2 O Glucose oxygencarbon dioxide water This reaction MUST be balanced!

Balancing Chemical Reactions Chemical reactions are balance by adding coefficients- numbers placed in front of the formula. –Never change the formula C 6 H (aq) + O 2(g) → CO 2 (g) + H 2 O Glucose oxygencarbon dioxide water Demonstrate on board. C 6 H (aq) + 6O 2(g) → 6CO 2 (g) + 6H 2 O Show products and reactants.

Writing / Balancing Chemical Equations 1. Write the equation (from a word problem) –Reactants on the left –Products on the right 2. Indicate physical state of reactants / products –(g) = gas –(l) = pure liquid –(s) = solid –(aq) = ion or molecular compound in aqueous (water) solution. 3. Balance equation so that the same number of atoms of each element are on the left as on the right. –Add coefficients in FRONT of chemical formula; NEVER change subscripts on formulas –Start by balancing an element that appears in only one species on each side. –USE TRIAL AND ERROR!

Types of Reactions –Synthesis –Decomposition –Single Replacement –Double Replacement Reactions involving water –Hydrolysis –Hydration –Dehydration Oxidation / Reduction –Combustion –Hydrogenation

Types of Reactions (Cont’) Synthesis – two or more elements or compounds combine to form one more complex compound. A + B → C Example: 2N 2 + O 2 → 2N 2 O Nitrogen can react with oxygen to produce dinitrogen monoxide (aka nitrous oxide – laughing gas)

Types of Reactions (Cont’) Decomposition (reverse of synthesis) break down of one compound to form elements or simpler compounds. AB → A + B Example: 2H 2 O →2H 2 + O 2 Under the proper conditions, water can decompose into hydrogen and oxygen.

Types of Reactions (Cont’) Single Replacement – one element trades places with a different element in a compound. A + BC → AC + B Example: Fe + CuSO 4 → FeSO 4 + Cu Iron reacts with a solution of copper sulfate producing iron sulfate and elemental copper.

Types of Reactions (Cont’) Double Replacement – two elements trade places with each other. AB + CD → AD + CB Example: NaCl (aq) + AgNO 3(ag) → NaNO 3(aq) + AgCl (s) When an aqueous solution of sodium chloride is mixed with silver nitrate, silver chloride precipitates leaving sodium nitrate in solution.

Reactions involving Water Hydrolysis – Water (hydro) splits (lyses) another molecule. H+ ester + water → carboxylic acid + alcohol –This reaction requires an acid catalyst. –Draw on board. Some drugs are catabolized (broken down) by hydrolysis –Procaine, chloroprocaine Catalyst – a substance that speeds up a reaction without itself being consumed or destroyed by the reaction.

Reactions involving Water (Cont) Hydration – addition of water across a double bond (alkene) in the presence of H+ (acid) catalyst. –Draw on board.

Reactions involving Water (Cont’) Dehydration – reverse of hydration H+/heat Alcohol → Alkene + water draw on board.

Oxidation / Reduction Reactions Oxidation / Reduction reactions are often called Redox reactions. –Oxidation – loss of electrons (e-) –Reduction – gain of electrons (e-) 2 Na (s) + Cl 2 (g) → 2 Na Cl (s) Types of oxidation / reduction Reactions –Combustion –Hydrogenation

Oxidation / Reduction Reactions (Cont’) Combustion – special type of oxidation / reduction reaction involving oxygen. CH 4(g) + 2O 2 → CO 2(g) + H 2 O Organic Chemist identify oxidation / reduction –An atom is oxidized if: Gains electrons OR –Attached to more oxygen in product than reactant Losses hydrogen –Is attached to fewer hydrogens in product than reactant –An atom is reduced if: Loses oxygen Gains hydrogen In the equation above, C is oxidized and oxygen is reduced. Many antiseptics are oxidizing agents. See p. 152

Oxidation / Reduction Reactions (Cont’) Hydrogenation – addition of H 2 across a double bond. Often requires a platinum (Pt) catalyst. –Draw on board.

Stoichiometry Stoichiometry – the quantitative relationship between reactants and products in a chemical reaction. Making cheese sandwiches. –If you have 20 slices of bread, how many sandwiches can you make? (2 slices of bread/ sandwich) 2 bread slices + 1 cheese slice = 1 sandwich Conversion factors = 1 cheese slice or2 bread slices 2 bread slices1 chees slice. Work problem on board

Stoichiometry (Cont’) Chemical Reactions (rxn) 2CO (g) + O 2 (g) → CO 2(g) carbonoxygencarbon monoxidedioxide Demonstrate with dozen, molecules, moles. Write conversion factors.

Stoichiometry (Cont’) Typical Question: If 6.0 mol CO is present, how many moles of O2 will be required to convert all of the CO to CO 2 ? Typical Question: If 6.0 mol CO is present, how many moles of CO2 will be produced if all of the CO is used up?

Real Life Example Firefighters us a device called a “rebreather” to enter smoky buildings. Rebreathers use KO 2 to remove carbon dioxide from air in the unit using the reaction below. 4 KO 2(s) + 2 CO 2(g) → 2 K 2 CO 3(s) + 3 O 2(g) How many grams of KO 2 are required to completely react with mol of CO 2 ? Work on board. ALWAYS GO THROUGH MOLES!

Yield of Reaction In the real world reactants are not always present in the exact ratio stated in the balanced chemical reaction. We need to discuss: –Limiting reagent –Theoretical Yield –Percent Yield

Limiting Reactant Limiting reactant – is the reactant that determines (limits) the amount of product that can be formed in a rxn. Balanced N 2 (g) + 3H 2(g) → 2NH 3(s) If 2.10 mol N 2 and 5.70 mol H 2 react, what is the limiting reagent. Ask the question, how many mol of H 2 is needed to react with.10 mol N 2 ? Work on board.

Theoretical Yield Theoretical yield – the maximum amount of product that can be obtained (controlled by amount of limiting reagent.) How to determine –1. Determine the limiting reagent. –2. Determine the theoretical yield. MUST Start with balance equation C 3 H 8(g) + 5 O 2(g) → 3 CO 2(g) + 4 H 2 O (g) If 2.2 mol propone (C 3 H 8(g) ) reacts with 14 mol oxygen, what is the theoretical yield? Two step problem 1. Calculate limiting reagent 2. Determine the theoretical yield of carbon dioxide (CO 2 )

Percent Yield Most real-world reactions do not produce the total theoretical yield, therefor we calculate percent yield to determine what the yield is % yield = actual yield X 100 theoretical yield

What determines if a Reaction will take place? Chemist classify reactions as: –Spontaneous – a reaction that continues by itself once it has started –Nonspontaneous – a reaction will not take place unless something starts it and keeps it going. Gibbs Free energy (G) determines if a reaction will take place (be spontaneous) or not take place (nonspontaneous) –Spontaneous reactions have a - ΔG –Nonspontaneous reactions have a + ΔG –Reactions in equilibrium have ΔG = 0

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O ΔG = kcal /mol Energy released. Energy is used to break and reform bonds.

Change in Free Energy (  G) A  B (Reactant) (Product) A B  EnergyEnergy Progress of Rxn.  A B  EnergyEnergy Progress of Rxn. 

Negative  G A  B (Reactant) (Product) A B  EnergyEnergy Progress of Rxn.  -  G spontaneous exergonic -energy released

Positive  G A  B (Reactant) (Product) +  G non-spontaneous endergonic - energy consumed A B  EnergyEnergy Progress of Rxn. 

Reactions Reaction Rate – how quickly products form Energy of activation – E a – the energy barrier that must be crossed to go from reactants to products. –Determines rate of reaction The greater Ea the slower the rate. Other factors influence rate –Temperature –Concentration of reactants –Presence of catalyst