Organic Chemistry I- Essential Terms

cis-trans isomers

(geometric isomers) Stereoisomers that differ in their cis-trans arrangement on a double-bond or on a ring. The cis isomer has similar group on the same side, and the trans isomer has similar groups on the opposite sides.

constitutional isomers

(structural isomers) isomers whose atoms are connected differently; they differ in their bonding sequence

Covalent bonding

bonding that occurs by the sharing of electrons in the region between two nuclei

single bond

a covalent bond that involves the sharing of one pair of electrons

double bond

a covalent bond that involves the sharing of two pair of electrons

triple bond

a covalent bond that involves the sharing of third pair of electrons

degenerate orbitals

orbitals with identical energies

delocalized charge

a charge that is spread over two or more atoms. We usually draw resonance forms to show how the charge can appear on each of the atoms sharing the charge

dipole moment (µ)

a measure of the polarity of a bond (or a molecule), proportional to the product of the charge separation times the bond length.

double bond

a bond containing four electrons between two nuclei. one pair of electrons forms a sigma bond, and the other pairs form a pi bond

electron density

the relative probability of finding an electron in a certain region of spacewithin an atom or molecule.

electronegativity

A measure of an element’s ability to attract electrons. Elements with higher electronegativities attract electrons more strongly

electrostatic potential map (EPM)

a computer-calculated molecular representation that uses color to show the charge distribution in a molecule. In most cases, the EPM uses red to show electron-rich regions (most negative electrostatic potential) and blue or purple to show electron-poor regions (most positive electrostatic potential). The intermediate colors orange, yellow, and green show regions with intermediate electrostatic potentials.

empirical formulas

the ratios of atoms in a compound

formal charges

a method for keeping track of charges, showing what charge would be on an atom in a particular Lewis Structure

geometric isomers

Stereoisomer that differ in their cis-trans arrangement on a double bond or on a ring

Hund’s rule

When there are two or more unfilled orbitals if the same energy (degenerate orbitals), the lowest-energy configuration places the electrons in different orbitals (with parallel spins) rather than pairing them in the same orbital

hybrid atomic orbital

A directional orbital formed from a combination of s and p orbitals on the same atom

sp hybrid orbitals give two orbitals with a bond angle of 180 degrees (linear geometry)

sp² hybrid orbitals give three orbitals with bond angles of 120 degrees (trigonal geometry)

sp³ hybrid orbitals give four orbitals with bond angles of 109.5 degrees (tetrahedral geometry)

ionic bonding

Bonding that occurs by the attraction of oppositely charged ions. Ionic bonding usually results in the formation of a large, three-dimensional crystal lattice

isomers

different compounds with the same molecular formula

• constitutional isomers (structural isomers) are connected differently; they differ in their bonding sequence

• cis-trans isomers (geometric isomers) are stereoisomers that differ in their cis-trans arrangement on a double bond or on a ring

• stereoisomers differ only in how their atoms are oriented in space

• stereochemistry is the study of the structure and chemistry of stereoisomers

isotopes

atoms with the same number of protons but different number of neutrons; atoms of the same element but with different atomic masses

LCAO

(linear combination of atomic orbitals) wave functions can add to each other to produce the wave function of new orbitals. the number of new orbitals generate equals the original number of orbitals

lewis structure

a structural formula that shows all valence electrons, with the bonds symbolized by dashes (-) or by pairs of dots, and nonbonding electrons symbolized by dots

line-angle formula

(skeletal structure, stick figure) a shorthand structural formula with bonds represented by lines. carbon atoms are implied wherever two lines meet or a line begins or bends. atoms other than C and H are drawn in, but hydrogen atoms are not shown unless they are on an atom that is drawn. each carbon atom is assumed to have enough hydrogens to give it four bonds

lone pair

a pair of nonbonding electrons

molecular formula

the number of atoms of each element in one molecule of a compound. the empirical formula simply gives the ratios of atoms of the different elements. For example, the molecular formula of glucose is C₆H₁₂O₆. Its empirical formula is CH₂O. Neither the molecular formula nor the empirical formula gives structural information

molecular orbital

an orbital formed by the overlap of atomic orbitals on different atoms. MOs can be either bonding or antibonding, but only the bonding MOs are filled in most stable molecules

• a bonding molecular orbital places a large amount of electron density in the bonding region between the nuclei. The energy of an electron in a bonding MO is lower than it is in an atomic orbital

• an antibonding molecular orbital places most of the electron density outside the bonding region. the energy of an electron in an antibonding MO is higher than it is in an atomic orbital

node

in an orbital, a region of space with zero electron density

nodal plane

in an orbital, a flat (planar) region of space with zero electron density

nonbonding electrons

valence electrons that are not used for bonding. a pair of nonbonding electrons is often called a lone pair

octet rule

atoms generally form bonding arrangements that give them filled shells of electrons (noble-gas configurations). for the second-row elements, this configuration has eight valence electrons

orbital

an allowed energy state for an electron bound to a nucleus; the probability function that defines the distribution of electron density in space. the Pauli exclusion principle states that up to two electrons can occupy each orbital if their spins are paired

organic chemistry

new definition: the chemistry of carbon compounds.

old definition: the study of compounds derived from living organisms and their natural products

pi bond

a bond formed by sideways overlap of two p orbitals. a pi bond has its electrons density in two lobes, one above and one below the line joining the nuclei

polar covalent bond

a covalent bond in which electrons are shared unequally. a bond with equal sharing of electrons is called a nonpolar covalent bond.

resonance hybrid

a molecule or ion for which two or more valid lewis structure