Chem 261 Vocabulary

Copy from mc on quizlet

electronegativity

A measure of the ability of an atom in a chemical compound to attract electrons

bond lengths

the distance between two covalently bonded nuclei

bond angle

the angle formed between two covalent bonds

bond dissociation energy

the energy required for homolytic cleavage of a bond (covalent bonds)

heterolytic cleavage

cleavage of a bond to give a cation and an anion; one atom which is part of the covalent bond retains both electrons from the bond

dispersion forces

everything has these; most important in nonpolar covalent bonds, strength of dispersion forces is related to molecular weight and polarizability

condensed structural formula

structural molecular formula showing the general arrangement of atoms but without showing all the covalent bonds

line-bond formula (aka skeletal structures)

a line is used to represent two electrons forming a bond (a shared pair)

condensed formula

bonds are always not shown and atoms of the same type bonded to another atom are grouped together

bond

in-phase waves; constructive interference (think "combine")

antibonding

out-of-phase waves; destructive interference (think "cancel")

node

point at which the amplitude is zero; shaded space within an orbital; no electron density here

antibonding orbital (𝜎*)

node between nuclei, zero electron density

bonding orbital (𝜎)

high electron density between nuclei

sp3 hybridization

tetrahedral, trigonal pyramidal, or bent

sp2 hybridization

trigonal planar

sp hybridization

linear

s orbital

p orbital

sigma bond

a bond formed when two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting the two atomic nuclei (there is always one sigma bond between two bonded atoms)

pi bond

a bond that is formed when parallel orbitals overlap to share electrons (any additional bond after a sigma!)

resonance structures

1. exist only on paper- the actual structures are hybrids of all the resonance structures

2. differ only in the position of electron pairs, not atoms

3. all structures should be proper lewis structures (a few exceptions)

4. all should have same number of unpaired electrons

hydrocarbons

compounds containing only hydrogen and carbon: alkanes, alkenes, and alkynes

generally not very reactive

isomer

compounds with the same molecular formula that differ in the order in which the atoms are bonded to one another; different chemical compounds with different chemical properties

physical properties of alkanes

1. nonpolar compounds

2. branching lowers the boiling point due to the disruption of LDFs (branching as in up/down connections vs a straight chain)

3. insoluble in water; soluble in organic solvents like diethyl ether, benzene

4. very unreactive

5. more carbons = higher BP

constitutional isomers

compounds having the same numbers of each type of atom but with a different order of attachment

properties of constitutional isomers

1. BPs and MPs increase with increasing size

2. branching decreases BPs and MPs but increases stability

Why do alcohols have higher boiling points than ethers?

Alcohols can H-bond, ethers can only have dipole-dipole interactions

complete combustion

A combustion reaction in which the only products are carbon dioxide and water

incomplete combustion

Combustion in which not enough oxygen is supplied to completely burn the fuel. Carbon monoxide is a common product or solid carbon (soot)

heat of combustion

energy released when a compound is completely oxidized to carbon dioxide and water; depends mostly on number of CH2 units

a lower heat of combustion means that the product is more stable: BRANCHED compounds tend to be more STABLE than their unbranched counterparts

steric energy

isolated molecule in gas phase at 0 K, relative energy of a conformation or stereoisomer calculated using classical mechanics

stretch

bond length; energy associated with stretching or compressing bonds from their optimal length

bend

bond angle; energy associated with deforming bond angles from their optimal angle

stretch-bend

energy required to stretch two bonds involved in a severly compressed bond angle

dipole-dipole

energy associated with interaction of bond dipoles

out of plane

energy required to distort a trigonal center out of planarity

torsional strain

destabilization from eclipsing of bonds on adjacent atoms

van der waals strain

destabilization from two atoms being too close together

dihedral angle

the angle between two specified groups in a Newman projection

dimensional projection

shows molecules in 3D

Newman projection

A method of visualizing a compound in which the line of sight is down a carbon-carbon bond axis.

staggered

a chemical conformation of an ethane-like molecule in which the substituents are at the maximum distance from each other

dihedral angle is 60°

eclipsed

two atoms/groups whose dihedral angle is zero, least stable configuration; highest in energy

anti

atoms/groups point in opposite directions, 180 dihedral angle

gauche

molecules (also usually of the same type) being 60° from each other on a Newman projection

ring strain

as the ring gets bigger, the strain energy goes down

equatorial bonds

Bonds on cyclohexane chair parallel to the ring

axial bonds

vertical and alternate above and below the ring

chair conformation of cyclohexane

most stable conformation of cyclohexane, lowest energy

half-chair conformation of cyclohexane

the unstable conformation halfway between the chair conformation and the boat conformation; highest energy

boat conformation of cyclohexane

least stable conformation of cyclohexane

twist boat

more stable than boat, less stable than chair, less stable than a half chair

conformations of cyclohexane

substituted cyclohexanes

preferentially occupy equatorial positions due to 1,3 diaxial interactions in axially substituted cyclohexanes

bicyclic compounds

compounds that contain two fused rings, conformationally "locked," chairs cannot flip back and forth between conformations, ex decalin

stereoisomers

compounds with the same structures differing only in their arrangement of atoms in space; not cis or trans

rules for assigning priority for stereoisomers (E/Z notation)

1. if the atoms are different, highest atomic number gets highest priority

2. if there are two isotopes of the same element, the one with the higher mass gets the higher priority

3. if the atoms are the same, the atomic numbers of the next atoms are used to assign priority

4. atoms attached by double or triple bonds are given single bond equivalencies

(E)

across, opposite sides of the bond

(Z)

together, priority groups are on on same sides of the bond

chiral

an object or molecule which cannot be superimposed on its mirror image; makes enantiomers

achiral

an object or molecule can be superimposed on its mirror image

enantiomers

isomers which are nonsuperimposable mirror images

stereogenic carbon atom

also called asymmetric center or chiral center; the carbon atom with four different groups bonded to it

fischer projections

-Horizontal lines indicate bonds that project out from the plane of the page

-Vertical lines indicate bonds going into the plane of the page

-point of intersection = carbon atom

properties of enantiomers

have almost all of the same physical and chemical properties, what differs is...

1. the interaction with other chiral substances

2. interactions with polarized light

optically active

compound that rotates the plane of polarized light

optical isomers

enantiomers

racemic mixture

A mixture that contains equal amounts of the (+) and (-) enantiomers; does not rotate a plane of polarized light; therefore optically inactive

Lower MP than pure enantiomers (each enantiomer depresses the other); designated by (±)

R-S notational system

1. assign priorities to the substituents according to CIP priority rules

2. orient the molecule with the lowest group pointing away (rear)

3. turn the wheel 1, 2, 3

4. if the direction of the semicircle is clockwise, the configuration is R. if it is counterclockwise, the configuration is S.

*remember, sp3 carbons are only chiral if all of their bonds are different

R

right, clockwise

S

left, counterclockwise

chiral compounds without chiral centers

diastereomers

stereoisomers that are not enantiomers; maximum number of 2^n isomers

have different physical and chemical properties from one another and are separable

meso compounds

contain an internal plane of symmetry and are achiral (mirror image is identical to the original); maximum number of 2^n -1 isomers

types of chiral tests

optical rotation, odor, physiological behavior

unequal mixtures

aka not racemic

characterized by their enantiomeric excess

enantiomeric excess = (% maj enantiomer) - (% minor enantiomer)