Ch 2: Thermochemistry

thermochemistry

allows us to study the relative energy of a molecule

strain

structural stress within a molecule that is not present in a reference compound

What makes a good reference molecule?

a completely different molecule without strain

same molecule with different orientation

internal energy

energy held within a molecule; potential energy

energy

ability to do work

reaction coordinate diagrams

allows us to study the relative energy of a molecule

Energy can be added to a system by...

mechanical means

thermal means

with light

Gibbs free energy

differences in the stability of two different combinations of an ensemble of molecules at standard state and pressure

Ideal gas law constant (R)

0.0821 L atm/mol K

8.314 J/(mol·K)

Exergonic

reactions that release energy, spontaneous in forward direction

Endergonic

reactions that absorb energy, spontaneous in reverse direction

Gibbs free energy equation

ΔG = ΔH - TΔS

Enthalpy

change in heat between two different compositions of molecules at constant pressure if no work is done

Exothermic

Releases heat

endothermic

Absorbs heat

Entropy

a measure of the disorder of a system

degrees of freedom

number of molecular movements possible

Degrees of freedom in a molecule

translations, rotational, vibrational

What impacts entropy?

number of molecules, temperature, volume

bond dissociation energy

energy required to break a bond

homolytic cleavage

bond cleavage that forms two radicals

heat of formation

The heat change that takes place when one mole of a compound in its standard state is formed from its elements in their standard states

heat of combustion

the heat of reaction for the complete burning of one mole of a substance

stability

denotes a lower internal energy relative to a reference system, intrinsic property, thermodynamically related

persistence

long-lived, kinetic-related

Radicals and Carbocations are stabilized with...

resonance and hyperconjugation

conformational analysis

allows us to determine the relationship between structure and energetics

conformers

stereoisomers that interconvert by rotations around single bonds

constitutional isomers

two molecules with same formula, different connectivity

staggered conformation

no overlap of atoms along the line of sight

lowest energy

eclipsed conformation

highest energy

no separation or 120 separation.

gauche conformation

a conformation with a 60 degree dihedral angle between the largest groups

anti conformation

A type of staggered conformation in which the two largest groups are antiperiplanar to each other

the most energetically favorable conformation

1,3-diaxial interaction

the strong steric strain between two axial groups on cyclohexane carbons with one carbon between them

chair conformation

The chair-shaped conformation of cyclohexane that has no angle strain and has no torsional strain because it is perfectly staggered about all the C-C bonds. It is strain free.

boat conformation

a nonplanar conformation of a cyclohexane ring in which carbons 1 and 4 of the ring are bent toward each other

conjugation

direct attachment of alkenes without intervening atoms

trans conformation is preferred

pi electrons prefer to be in a _____ orientation

planar

Huckle Rule

any hydrocarbon or heterocycle with 4n+2 pi electrons in a fully conjugated cyclic pi-system is aromatic

Homoaromaticity

molecule whose geometry allows for orbital overlap, but has a saturated center

Anti-aromatic

planar pi-systems with 4n electrons

generally unstable

orbital effects

orbitals will orient to lower internal energy

anomeric effect

lone pairs of glycosytic bond donates into anti-bonding orbital of adjacent functional group