Ch 4 - Analyzing Organic Reactions Flashcards
(40 cards)
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
