Orgo - nomenclature, bonding, isomers Flashcards
nucleophiles
Nucleophiles are defined as “nucleus-loving” species with either lone pairs or π bonds that can form new bonds to electrophiles. You may have noted that nucleophilicity and basicity appear to have similar definitions—and this is true! Good nucleophiles tend to be good bases. There is, however, a distinction between the two. Nucleophile strength is based on relative rates of reaction with a common electrophile—and is therefore a kinetic property. Base strength is related to the equilibrium position of a reaction—and is therefore a thermodynamic property
Nucleophiles tend to have lone pairs or π bonds that can be used to form covalent bonds to electrophiles. On Test Day, look for carbon, hydrogen, oxygen, or nitrogen (CHON) with a minus sign or lone pair to identify most nucleophiles.
nucleophilicty based on which factors
Nucleophilicity is determined by four major factors:
Charge: Nucleophilicity increases with increasing electron density (more negative charge)
Electronegativity: Nucleophilicity decreases as electronegativity increases because these atoms are less likely to share electron density
Steric hindrance: Bulkier molecules are less nucleophilic
Solvent: Protic solvents can hinder nucleophilicity by protonating the nucleophile or through hydrogen bonding
polar protic solvents and nucleophilicty
in polar protic solvents, nucleophilicity increases down the periodic table
In protic solvents, nucleophilicity decreases in the order:
I– > Br– > Cl– > F–
This is because the protons in solution will be attracted to the nucleophile. F– is the conjugate base of HF, a weak acid. As such, it will form bonds with the protons in solution and be less able to access the electrophile to react. I–, on the other hand, is the conjugate base of HI, a strong acid. As such, it is less affected by the protons in solution and can react with the electrophile.
polar aprotic solvents and nucleophilicty
In polar aprotic solvents, nucleophilicity increases up the periodic table
In aprotic solvents, on the other hand, nucleophilicity decreases in the order:
F– > Cl– > Br– > I–
This is because there are no protons to get in the way of the attacking nucleophile. In aprotic solvents, nucleophilicity relates directly to basicity.
We won’t use nonpolar solvents with this type of reaction because we need our nucleophile to dissolve. Because charged molecules are polar by nature, a polar solvent is required to dissolve the nucleophile as well because like dissolves like . Examples of strong nucleophiles include HO−, RO−, CN−, and N3−. NH3 and RCO2− are fair nucleophiles, and H2O, ROH, and RCOOH are weak or very weak nucleophiles. As far as functional groups go, amine groups tend to make good nucleophiles.
An acid–base reaction will only proceed if the products that will be formed (the conjugate base of the acid and the conjugate acid of the base) are weaker than the original reactants.
T/F?
T
protic vs aprotic solvents
electrophiles
Electrophiles are defined as “electron-loving” species with a positive charge or positively polarized atom that accepts an electron pair when forming new bonds with a nucleophile. Again, this definition brings to mind Lewis acids. The distinction, as with nucleophiles and bases above, is that electrophilicity is a kinetic property, whereas acidity is a thermodynamic property. Practically, however, electrophiles will almost always act as Lewis acids in reactions. A greater degree of positive charge increases electrophilicity, so a carbocation is more electrophilic than a carbonyl carbon. Some comparisons between electrophiles are drawn in Figure 4.4. Additionally, the nature of the leaving group influences electrophilicity in species without empty orbitals; better leaving groups make it more likely that a reaction will happen. If empty orbitals are present, an incoming nucleophile can make a bond with the electrophile without displacing the leaving group.
electrophilicty ranking
carboxylates are least reactive
