Reactivity Rules Electronegativity - The measure of an atoms’ affinity for electrons - Asses high and low electron density 0.4 below - Non-Polar covalent 0.4 - 1.7 - Polar covalent 1.7 above - Ionic Carbon - Central atom - All the metals are less electronegative than carbon, whereas the non-metals are more electronegative. (w/ exceptions) Covalent bond - Bonds b/w atoms consisting of a shared electron pair - Not shared equally unless they have the same exact electronegativity. E.g C - C = 0 because it’s a bond between two same elements. - Element with the highest electronegativity will exert the strongest pull on the electrons of the shared bonds. Result: high electron density/polarized. Ionic bond - An atom donates an electron to the other atom, thus creating a cation and an anion. Important: Location of charges - positive or negative charges have a big impact on the electron density. But, molecules with formal charges are not always located on the atom you think it is and is sometimes spread out over several atoms. Lone-pairs - pair of electrons that do not partake in bonding (count valence electrons and bonds; remaining pairs are lone pairs) Pi-electrons - present in double and triple bonds. Electrons in pi-orbitals have a higher energy level than electrons in sigma-orbitals. Pi-electrons are easily broken, either to form new bonds or to place a negative charge on one of the bonding atoms. Polarity - (-) higher EN; gives electrons more - (+) lesser EN; receives electrons Reaction Elements: Nucleophiles (Has money to spend) - Nucleophilic (nucleus loving) - Has electrons to spare. - Reaction begins Fast in addition or substitution reactions 3 key factors that influence nucleophiles: Charge, Electronegativity, and Size. Negative (-) charged atoms are stronger nucleophiles. Less electronegativity, the better. The larger nucleophiles, the harder it is for them to attack the electrophile because there isn’t room (AKA Steric hindrance). Basicity is a subset of nucleophilicity, where the nucleophile donates electrons specifically to a proton. Solvency is the fourth factor. Electrophiles (Has something to sell) - Electron-loving. This is where the nucleophile will react. - Accepts an electron pair. - Nucleophiles: bases : Electrophiles: acids - Which is more electrophilic? Carbon = Oxygen (+); Electrophile: Carbon. Because Carbon (2.5) is less electronegative than the Oxygen (3.5) atom (AKA Resonance concept). - Electrophiles are areas in a molecule that have a relatively low electron density. With this, they readily accept electrons from nucleophiles to form new bonds. Leaving groups - Leaves the molecule, breaks a bond, and takes a pair of electrons with it. - Have a high electron density around it -- an element that is highly electronegative. - Weak bases are good leaving groups. The stronger the corresponding acid, the better the leaving group (the corresponding base of a strong acid is a weak base). Remember: Electrons move from an area of high electron density to an area of low electron density and form a new bond. Arrows that form bonds (lone-pair to bond) Arrows that break bonds (bond to lone-pair) Arrows that form and break bonds at the same time (bond to bond) Organic Compounds Reaction Types: Addition - A new part is added to a substrate molecule, but no part of it is broken off. - Double or triple bonds. - Nucleophilic: aldehydes and ketones - Electrophilic: alkenes and alkynes Substitution - A bond is made and a bond is broken. - Contains at least a leaving group. Nucleophilic - new substituent is a nucleophile - Nucleophilic (SN1/SN2): Alkyl Halides, Alcohols - Nucleophilic: Carboxylic Acids & Derivatives Electrophilic: new substituent is an electrophile - Electrophilic: Aromatic compounds Elimination - No new bonds are formed in the substrate. Bonds are broken. - The substrate could be similar to the substitution reaction. - Elimination (E1/E2): Alkyl Halides, Alcohols
