Introduction to Organic Compounds Flashcards
define organic chemistry
the study of compounds containing carbon
why is carbon the most abundant element in living things?
- unique bonding properties and forms covalent bonds with a wide diversity of elements
- each carbon atom may form up to 4 covalent bonds
- can form chains, rings, and complex molecules
- carbon compounds show remarkable variation in structure: may be flat or 3D
define hydrocarbons
organic compounds made up of only carbon and hydrogen
alkanes
- only C-C bonds
- general formula CnH2n+2
alkenes
- contain a C=C bond
alkynes
- contain a C≡C bond
state the four kinds of orbitals for electrons, arranged from lowest energy to highest energy
s, p, d, and f
where is the probability of finding an electron highest?
near the nucleus; the probability decreases as the distance from the nucleus increases
shape of an s orbital
spherical
shape of p orbitals
- dumbbell shapes of two lobes separated by a region of zero electron density (a node)
- lobes are either + or - (one orbital is assigned a positive phase, the other negative)
- 2px, 2py, and 2pz are perpendicular and equal energy (degenerate)
rules for filling the ground-state electron configuration (lowest energy arrangement)
- lowest energy orbitals fill first (1s->2s etc)
- maximum of two spin paired electrons can occupy an orbital (Pauli exclusion principle), with one electron up and the other down
- if two or more empty orbitals of equal energy are available, electrons occupy each one with spins parallel until all orbitals have one electron
why do valence electrons participate in bonding?
as they are less stable
why do bonds form?
because molecules have a lower energy than individual atoms
ionic bonding
- electrostatic attraction between oppositely charged ions
- bond forms from electron transfer
covalent bonding
- sharing of electrons between atoms
- each bonding atom usually contributes one electron to the new bond (opposite spin)
sigma bond
- type of covalent bond
- head on orbital overlap
- s-s, s-p, p-p overlap
pi bond
- type of covalent bond
- side-to-side orbital overlap
- p-p orbitals
electronegativity
the ability of an atom to pull electrons toward itself through a bond
give the formula for Formal charge
FC = (group #) - (# of bonds) - (# non-bonded electrons)
define VSEPR
Valence Shell Electron Pair Repulsion Theory
the most stable structure is the one which valence electron pairs (bonding pair or lone pair of electrons) are as far apart as possible to minimise electron-electron repulsion
3 possible structural geometries of carbon
- tetrahedral
- trigonal planar
- linear
tetrahedral
- four groups around carbon are equally distributed in a pyramid arrangement
- bond angle of about 109’ between atoms
trigonal planar
- three groups around carbon are equally distributed in the same plane (flat)
- bond angle of about 120’ between atoms
linear
- two groups around carbon are equally distributed in a line
- bond angle of 180’ between atoms
Valence bond theory
covalent bond involves the sharing of two spin-pasted electrons through the overlap of atomic orbitals
better orbital overlap = stronger bond
define hybrid orbitals
atomic orbitals that mix to give the observed geometry of atoms
why are hybrid orbitals necessary to explain how carbon bonds to other elements?
- if unhybridised, 4 bonds would be made with 4 different orbitals
- would end up with different bond lengths and angles would be 90, which is inconsistent with experimental values
sp3 hybrid orbitals
- one 2s orbital and three 2p orbitals combine to form four equivalent tetrahedral orbitals
- four sp3 orbitals are arranged with a bond angle of 109’
use methane as an example of sp3 orbitals
- each sp3 orbital on C overlaps with a 1s orbital on H
- head-on orbital overlap forms a C-H sigma bond
- each C-H bond is identical (same length)
- each H-C-H bond angle is 109’
sp2 hybrid orbitals
- one 2s orbital and two 2p orbitals combine to form three equivalent trigonal planar orbitals
- three sp2 orbitals have a bond angle of 120
- one unhybridised 2px orbital is perpendicular to the plane of the sp2 hybrids
use formaldehyde (CH2O) as an example of sp2 orbitals
- two C-H sigma bonds formed by overlap of sp2 orbital on C with 1s orbital on H
- one C-O sigma bond is formed by overlap of sp2 orbital on C with a sp2 orbital on O
- double bond between C and O formed by a side-by-side orbital overlap of unhybridised 2pz orbitals (pi bond)
sp orbitals
- one 2s orbital and one 2p orbital combine to form two equivalent linear orbitals
- two sp hybrid orbitals with a bond angle of 180
- two unhybridised 2p orbitals perpendicular to the plane of the sp hybrids and each other
use acetylene (C2H2) as an example of sp hybrid orbitals
- two C-H sigma bonds formed by overlap of sp orbital on C with a 1s orbital on H
- one C-C sigma bond formed by overlap of sp orbitals on each C
- two C-C pi bonds formed through side-by-side orbital overlap of two 2py orbitals and two 2pz orbitals
sp3 hybridisation of nitrogen
sp3 hybridisation of oxygen
shortcut for determining hybridisation
no of groups around atom, no of orbitals used, type of hybrid orbital, bond angle, geometry
- 4, 4, sp3, 109.5, tetrahedral
- 3, 3, sp2, 120, trigonal
- 2, 2, sp, 180, linear
constitutional isomers
molecules with the same molecular formula but different bonding
define resonance
used to describe the bonding in some bonding situations which cannot be described with a single structure due to the special way pi orbitals interact
define a delocalised pi bond
sharing of 2 electrons among more than 2 atoms
why does delocalisation take place?
sharing of electrons among many atoms (delocalisation) stabilises molecules
three characteristics of resonance forms
- identical placement of atoms
- different placement of electrons (lone pairs and pi electrons)
- a double-headed arrow separating them
what is a resonance hybrid?
the actual structure of the molecule, a blend of all the resonance forms that show the delocalised electrons
for a molecule to have resonance, it needs to have at least one of the following:
- pi bond with atoms of different electronegativities
- pi bond beside an atom with a p orbital (eg paired or unpaired electrons, pi bonds)
- atom with incomplete octet next to an atom with a lone pair of electrons
give an example of pi bond with atoms of different electronegativities (resonance forms)
give an example of a pi bond directly beside paired electrons (resonance forms)
give an example of a pi bond being directly beside an atom with an incomplete octet
give an example of pi bonds being directly beside other pi bonds
give an example of an atom with an incomplete octet next to an atom with a lone pair of electrons
structures cccan have more than 2 resonance forms if
more than 3 atoms in sequence have p orbitals
do resonance structures contribute equally to the resonance hybrid?
no; forms with favourable electron distribution are stronger contributors
define a functional group
an atom or specific groups of atoms which give molecules distinct reactivity and properties
hydrocarbons
- alkane
- alkene
- alkyne
organohalide
X = F, Cl, Br, I
amine
alcohol
ether
nitrile
types of carbonyl groups
- ketone
- aldehyde
- carboxylic acid
- ester
- amide
define the inductive effect
the distribution of electron density in a bond due to differences of electronegativity
describe the dipole moment
- a measurable property of a molecule
- describes a separation of charge in a molecule as a result of the vector sum of all bond dipoles
- C-H bonds are considered non-polar
define electrostatic interactions
when oppositely charged molecules attract
define dipole-dipole interactions
attractive forces between the negative end of a permanent dipole in a molecule and the positive end of a permanent dipole in a neighbouring molecule
define hydrogen bonding
the attractive force between a N or O lone pair and a H in OH or NH
define and describe dispersion forces
- attractive interactions that exist between all molecules in close proximity to each other
- result from small temporary dipoles induced in each molecule by the other
- particularly important for non polar molecules
boiling and melting points are a reflection of
relative strength of intermolecular forces
as alkane size increases, mp and bp
increase due to increasing dispersion forces
polar groups impart higher mp/bp values due to
increased dipole-dipole interactions (strong), hydrogen bonding (stronger), or electrostatic effects (strongest)
state the three types of solvent
- polar protic solvents
- polar aprotic solvents
- non polar solvents
polar protic solvents
H-bond donors, eg OH, NH, very polar
polar aprotic solvents
strong dipoles (polar), most are H-bond acceptors
non polar solvents
mainly non polar bonds or no significant net dipole
what type of solvent is ether
non polar
describe how ‘like dissolves like’ works in organic chemistry
- strongly polar solvents dissolve strongly polar/ionic substances
- weakly polar solvents dissolve weakly polar/nonpolar substances)
‘many organic molecules are polar or weakly polar and dissolve in
polar solvents
highest quality resonance forms have the following characteristics
- most atoms with full octets
- fewest number of formal charges
- if formal charges exist, the negative charges are on the most electronegative atoms while positive charges on the most electropositive atoms
- like charges separated by the maximum distance possible, while opposing charges as close together as possible
acyl group
acetyl
formyl
benzoyl
