Chapter 5: An overview of organic reactions 5.1 Kinds of organic reactions Even though there are hundreds of reactions to study, 1. Addition: combination of two molecules into one. organic chemistry is governed by only a few key ideas that determine chemical reactivity.
First, reactions can be organized by what kinds of reactions occur. Then, we can study how those reactions occur. 2. Elimination: one molecule splits into two.
Both need to happen in order to fully understand organic chemistry.
Four general types of organic reactions: 1. Addition 3. Substitution: two molecules exchange parts to give 2. Elimination two new products. 3. Substitution 4. Rearrangement
4. Rearrangement : a single reactant rearranges its atoms to give an isomeric product.
ch5 Page 1 ch5 Page 2 5.2 How organic reactions occur: Mechanisms 5.3 Radical reactions In a clock, we see the hands move but the mechanism Radical : highly reactive molecule or atom with an behind the face is what causes the movement. odd number of electrons (usually 7) in its valence shell (instead of the stable octet). In an organic reaction, we see the transformation that has occurred. The mechanism describes the steps that Some example radicals: cause the changes we observe.
Mechanisms for organic reactions are the series of steps in a reaction sequence from reactant to product Bond -making or bond -breaking Steps can occur one at a time or at the same time (concerted ) Fishhook arrow shows the movement of only one Bond formation or breakage can be symmetrical or electron. unsymmetrical.
Homolytic : symmetrical - electrons move one at a time - radical reactions Radical substitution:
Rad ∙ + A : B →
Radical addition: Heterolytic : unsymmetrical - electrons move as a pair - polar reactions Rad ∙ + →
ch5 Page 3 ch5 Page 4 Steps in radical substitution 5.4 Polar reactions There are three types of steps in a radical substitution Differences in electronegativities make electron -rich and reaction: electron-poor regions in any polar molecule.
1. Initiation : homolytic formation of two reactive species with unpaired electrons - often initiated by light ( hν)
2. Propagation : reaction of radical with molecul3e to Bond polarity can be increased by acid -base reactions: make a new radical.
3. Termination : combination of two radicals to form a stable product. Even though electronegativities are similar between C -S and C-I, these bonds are polar because the electrons in the large S and I atoms are polarizable - they can easily respond to other nearby charges.
ch5 Page 5 ch5 Page 6 Nucleophiles and electrophiles Nucleophiles and electrophiles Because unlike charges attract , electron -rich sites seek out and react with electron-poor sites.
Nucleophile : electron -rich ( - or δ-) atom that seeks out an electron-poor atom (nucleus-loving) - Must have lone pair of electrons! Nucleophiles are Lewis bases .
Electrophile : electron -poor (+ or δ+) atom that receives electrons from a nucleophile (electron-loving) Electrophiles are Lewis acids .
When drawing a mechanism, a curved double - headed arrow is used to show the movement of a pair of electrons, from nucleophile to electrophile (Never the other way around!!) When the electrophile is an H, it's an acid -base reaction!
- + CH 3O + H 3O →
ch5 Page 7 ch5 Page 8 5.5 Addition of HBr to ethylene Mechanism of addition of HBr to ethylene Finding the nucleophile and electrophile is the key to The mechanism of this reaction happens in two steps: almost every organic reaction. Let's start with a simple addition reaction. 1. The π electrons from the nucleophilic double bond attack the electrophilic hydrogen on HBr, forming a new C -H σ bond.
This leaves the other carbon (formerly of the π bond) with only 6 electrons and an empty p orbital - a positively charged carbocation.
Simultaneously, two electrons from the H -Br σ bond move onto bromine, making a bromide ion.
2. The bromide ion donates an electron pair to the + carbocation, forming a C-Br σ bond and yielding the Ethylene contains an electron -rich π bond neutral addition product. The pair of electrons can act as a nucleophile when there is a strong acid present.
HBr is a strong acid and contains an electron -poor H. The δ+ H acts as a strong attractor for electrons from another molecule. The H is the electrophile in this reaction.
ch5 Page 9 ch5 Page 10 5.6 Using curved arrows in polar reaction mechanisms Using curved arrows in polar reaction mechanisms Practice and the knowledge of a few rules will help you 3. The electrophile can be either positively charged or draw correct curved arrows for reaction mechanisms. neutral. i. Positive electrophiles become neutral products. 1. Electrons always move from a nucleophile (Nu: or Nu: -) to an electrophile (E or E +). Always start with an electron pair! ii. Neutral electrophiles become negative products. Electrons usually flow from one of these nucleophiles:
4. Never exceed a first - or second -row atom's octet. Carbon never makes more than four bonds! Electrons usually flow to one of these electrophiles: Hydrogen never makes more than one bond!
2. The nucleophile can be either negatively charged or neutral. i. Negative nucleophiles become neutral products.
ii. Neutral nucleophiles become positive products.
ch5 Page 11 ch5 Page 12 5.7 Equilibria, rates, and energy changes Free energy and equilibrium Reactions can move either forward or reverse to reach In order for a reaction to proceed to completion, the equilibrium. products must be lower in potential energy than the reactants. Recall from general chemistry that the equilibrium constant, Keq , is the ratio of product concentrations over (Reactive high -energy molecules lose energy until they reactant concentrations (each raised to the power of the become more stable low-energy molecules.) balancing coefficient) This change of potential energy during a reaction is called G For aA + bB ⇌ cC + dD, the Gibbs free energy change (Δ ). o For Keq > 1, ΔG must be negative (the system releases energy to become more stable). For the reaction we just studied, there is a very large This is a spontaneous process. o equilibrium constant. For Keq < 1, ΔG must be positive (the system would have to gain potential energy and become less stable in order to proceed as written)
ΔG is influenced by two energetic properties: enthalpy and entropy. ΔG = ΔH - TΔS Enthalpy change ( ΔH) is related to the strength of Keq > 1000 means the reaction goes to completion - the bonds that are broken and formed. A favorable amount of unreacted starting material will usually be enthalpy change will have stronger (more stable) undetectable. (The product concentration is 1000x the bonds in the product. reactant concentration at equilibrium) Entropy change ( ΔS) is related to freedom of motion and dispersion of energy. Elimination: A → B + C: favorable entropy change Addition: A + B → C: unfavorable entropy change
ch5 Page 13 ch5 Page 14 5.8 Bond dissociation energies 5.9 Energy diagrams and transition states Whenever a bond is formed, energy is released (like the In the reaction of HBr and ethylene, the first step is sound made when two magnets hit each other). formation of the carbocation.
Whenever a bond is broken, energy is absorbed (like the force required for you to pull two magnets apart).
Bond dissociation energy ( D): amount of energy required to break a bond to produce two radical fragments: The highest energy, most unstable point in one step of a reaction is called the transition state .
The transition state occurs when the C -H bond is partially This energy is mostly dependent of the type of bond, not formed and the H-Br bond is partially broken. the molecule that the bond is in.
Among the weakest bonds are those that can readily produce radicals: Cl -Cl 242 kJ/mol Br -Br 194 HO -OH 211
σ bonds become stronger the more s character they have
CH 3-H 439 kJ/mol
H2C=CH -H 464 HC ≡C-H 558
ch5 Page 15 ch5 Page 16 Activation energy and free energy Kinetics vs. equilibrium The energy needed to go from reactant to transition Activation energy determines kinetics. state is the activation energy (ΔG‡). Gibbs free energy change determines equilibrium.
The size of the activation energy determines the rate of the reaction (whether it will occur quickly or slowly). A high ΔG‡ means very few of the molecules in the sample will have enough energy to reach the transition state and the reaction will be slow . A low ΔG‡ means most of the molecules will already have enough energy to reach the transition state and the reaction will be fast at room temperature.
The energy difference between the reactant and the product is the standard free energy difference, ΔGo - as we saw before, this determines what the equilibrium A reaction with a very high activation energy may never constant will be. reach its equilibrium mixture of products because it is too slow! (The combustion of gasoline has a large equilibrium constant - the products are favored energetically - but at room temperature, the reaction is too slow to observe.) Solution: heat the reaction so more molecules have enough energy to reach the transition state Solution: use a catalyst so there is a series of different, lower-energy transition states.
ch5 Page 17 ch5 Page 18 5.10 Intermediates An intermediate is the product of one step of the reaction, and reactant of the next step.
In the complete energy diagram, intermediates are minima in the curve, while transition states are maxima in the curve.
Catalysts and enzymes make reactions faster by changing the mechanism to have several lower -energy transition states
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