Ch 4 - Analyzing Organic Reactions Flashcards
What are Lewis acids and bases?
- acids: electron acceptors; have vacant orbitals or positively polarized atoms
- bases: electron donors; have a lone pair of electrons and are often anions
What are Bronsted-Lowry acids and bases?
- acid: proton donors
- bases: proton acceptors
What are amphoteric molecules?
- can act as either acids or bases, depending on reaction conditions
- water is a common example as well as bicarbonate and dihydrogen phosphate
What is the acid dissociation constant, Ka?
- a measure of the acidity
- the equilibrium constant corresponding to the dissociation of an acid, HA, into a proton (H+) and its conjugate base (A-)
Ka = [H+][A-]/[HA]
What is pKa and its periodic trend?
- the negative logarithm of Ka (pKa = -logKa)
- a lower (or even negative) pKa indicates a stronger acid
- decreases down the periodic table and increases with electronegativities
What are common acidic functional groups?
- alcohols, aldehydes, ketones, carboxylic acids and their derivatives
- an alpha-hydrogen (hydrogens connected to an alpha-carbon, a carbon adjacent to a carbonyl) are acidic
What are common basic functional groups?
amines and amides
What are nucleophiles?
- nucleus loving and contain lone pairs or pi bonds
- have increased electron density and often carry a negative charge
How is nucleophilicity relate to basicity?
it is similar however, nucleophilicity is a kinetic property, while basicity is thermodynamic
What can affect nucleophilicity?
- charge: n increases with increasing electron density (more negative charge)
- electronegativity: n decreases as EN increases because these atoms are less likely to share electron density
- steric hindrance: bulkier molecules are less N
- solvent: protic solvents can hinder N by protonating the nucleophile through H bonding
What are common organic nucleophiles?
amino groups
- anions: want to form bonds within nearby positive charge
- double/triple bonds: have extra electron within pi bond and increased electron density
- lone pairs
- good base
What are electrophiles?
- electron loving and contain a positive charge or positively polarized
- more positive compounds are electrophilic
- can accept electrons to make bonds
What are common electrophiles?
alcohols, aldehydes, ketones, carboxylic acids and their derivatives
What are leaving groups and makes a good leaving group?
- molecular fragments that retain the electrons after heterolysis
- the best-leaving groups can stabilize additional charge through resonance or induction: will dissociate along with the electron in its bond and be more stable than nucleophile before it reacted
- weak bases (the conjugate bases of strong acids) make good leaving groups, halogens (especially further down group), inorganic esters, water, alcohol, ethers, tosylate ion
- alkanes and hydrogen ions are almost never leaving groups because they form reactive anions (strong bases, carbanions)
- nucleophile must be stronger base than leaving group
What are the steps followed for unimolecular nucleophilic substitution (Sn1) reactions to proceed?
- in the first step, the leaving group leaves, forming a carbocation, an anion with a positively charged carbon atom
- in the second step, the nucleophile attacks the planar carbocation from either side, leading to a racemic mixture of products
What do Sn1 reactions prefer?
more substituted carbons because the alkyl groups can donate electron density and stabilize the positive charge of the carbocation
What does the rate of Sn1 reactions depend on?
only on the concentration of the substrate: rate = k[R-L]
What are the steps followed for biomolecular nucleophilic substitution (Sn2) reactions to proceed?
- the nucleophile attacks at the same time as the leaving group leaves
- the nucleophile must perform a backside attack, which leads to an inversion of stereochemistry
- the absolute configurations is changed - R to S and vice versa - if the incoming nucleophile and the leaving group have the same priority in the molecule
What do Sn2 reactions prefer?
less substituted carbons because the alkyl groups create steric hindrance and inhibit the nucleophile from accessing the electrophilic substrate carbon
What does the rate of Sn2 reactions depend on?
the concentration of both the substrate and the nucleophile: rate = k[Nu:][R-L]
What is the oxidation state and which have the highest and lowest states?
- the charge an atom would have if all its bonds were completely ionic
- carboxylic acids and their derivatives are the most oxidized functional groups; followed by aldehydes, ketones, and imines; followed by alcohols, alkyl halides, and amines
What compounds have the lowest and highest oxidation state of carbon?
CH4 is the lowest oxidation state of carbon (most reduced); CO2 is the highest (most oxidized)
What is oxidation?
an increase in oxidation state and is assisted by oxidizing agents
What are oxidizing agents?
- accept electrons and are reduced in the process
- have a high affinity for electrons or an unusually high oxidation state
- often contain a metal and a large number of oxygens
What can primary alcohols be oxidized to?
- aldehydes by pyridinium chlorochromate (PCC)
- carboxylic acids by strong oxidizing agents like chromium trioxide (CrO3) or sodium or potassium dichromate
What can secondary alcohols be oxidized to?
ketones by most oxidizing agents
What can aldehydes be oxidized to?
carboxylic acids by most oxidizing agents
What is reduction?
a decrease in oxidation state and is assisted by reducing agents
What are reducing agents?
- donat electrons and are oxidized in the process
- have low electronegativity and ionization energy
- often contain a metal and a large number of hydrides
What can aldehydes, ketones, and carboxylic acids be reduced to?
alcohols by lithium aluminum hydride
What can amides and esters be reduced to?
- amides –> amines by LiAlH4
- esters –> a pair of alcohols by LiAlH4
When will an acid-base reaction proceed, based on the strength of the reactants and products?
acid-base reactions will proceed when the acid and base react to form conjugates that are weaker than the reactants
How do the definitions of nucleophile and electrophile differ from those of Lewis base and acid?
- N and E are based on relative rates of reactions and are therefore kinetic properties
- A and B are measured by the position and equilibrium in a protonation or deprotonation reaction and are therefore thermodynamic properties
How must the nucleophile and leaving group be related in order for a substitution to proceed?
a substitution reaction will proceed when the nucleophile is a stronger base (more reactive) than the leaving grou
Will an Sn1 reaction occur faster in tertiary or secondary carbons?
- tertiary where a carbocation can be most easily stabilized
What are the 6 steps for solving organic chemistry reactions?
- know nomenclature
- identify functional groups
- identify other reagents
- identify most reactive functional groups
- identify the first step of the reaction
- consider stereoselectivity
If there are no reaction conditions listed, what determines how the reaction will proceed?
the properties of the functional groups on the reactants themselves
Why do Sn1 reactions show first-order kinetics?
because the rate-limiting step involves only one molecule
Why are aldehydes generally more reactive than equivalent ketones to nucleophiles?
- difference in steric hindrance
- aldehydes have one alkyl group connected to the carbonyl carbon, wheras ketones have 2 creating more steric hindrance
What is the hierarchy to the reactivity of carboxylic acid derivatives toward nucleophiles?
anhydrides > carboxylic acids and esters > amides
- higher can form lower, but not vice versa
