Test 1 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
define degree of unsaturation
the number of pi bonds and/or rings in a molecule
give the formula for degree of unsaturation
[(2C + 2) - H + Group 5 - Group 7]/2
function of Mass Spectrometry
gives information about the mass of a compound and the fragments from which it is formed
describe how a typical mass spectrometer functions
- molecules are ionised (made into charged species) and fragmented
- ions are separated based on mass depending on their mass-to-charge ratio
what does a mass spectrum show?
- detected ion masses (m/z) and their relative abundance
- most important peak is the molecular ion peak (usually the heaviest ion in large abundance, M+) as it represents the molecular mass of the compound
define spectroscopy
the measurement of the interaction between a molecule and electromagnetic radiation
what is the effect of infrared radiation?
it causes excited stretching and bending vibrations (oscillations) of bonds that contain a dipole (no dipole = no vibration)
four types of stretching caused by IR
- symmetrical stretching
- asymmetrical stretching
- in-plane bending
- out-of-plane bending
describe how frequency of vibration is related to bond strength and atom weight
stronger bonds and lighter atoms vibrate at higher frequencies (higher energies)
draw a diagram for an IR spectrometer
consists of a high-quality infrared light source, a slit to create a parallel beam, a sample carrier, and a detector
- there is an incident IR light beam
- light is absorbed by the compound only at its vibration frequencies
- IR light absorbed by the compound is absent at the detector, showing up as IR peaks
describe the appearance of a typical IR spectrum
different bonds vibration at different energies which give rise to unique absorption bands with characteristic intensity, shape and frequency, stated as wavenumber
state the 4 main regions of a typical IR spectrum
from left to right:
- hydrogen region
- triple bonds
- double bonds
- fingerprint region (less useful)
energy and frequency increase from right to left
IR spectra: alkanes primarily give
C-H stretches (sp3)
IR spectra: alkenes primarily give
C-H stretches (sp2) and C=C stretches
IR spectra: alkynes primarily give
C-H stretches (sp) and C≡C stretches
IR spectra: aromatics primarily give
C-H stretches (sp2) and C=C stretches
what is a defining characteristic of Oh bands and NH bands
they are broad at 2700 - 3600 cm-1
are OH or NH bands stronger
OH bands are broader (more H bonding is occurring) and stronger than NH bonds (the more polar the bond, the stronger the signal)
carbonyl groups produce
strong and sharp C=O bands
how does NMR work?
some atomic nuclei, like protons (1H), behave like spinning spheres. since the nuclei (positively charged) have electrons (negatively charged) surrounding then, a small local magnetic field is created when they spin (magnetic moment).
- in the absence of an external magnetic field, the magnetic moments are randomly oriented
- when an external magnetic field (B0) is applied, the magnetic moments align, with some opposed to and some parallel to B0
3 steps by which NMR works
- the population of nuclei in the lower (more stable state) is slightly greater
- electromagnetic radiation causes some nuclei to become excited from a lower to higher E state (spin-flip) the frequency at which this spin flip occurs is called the resonance frequency
- as the nuclei relax back, they emit a signal that provides information about their unique chemical environment
magnetic field strength of small organic molecules vs large biomolecules
small: 300-700 MHz
large: 700+ MHz
what information does a HNMR spectrum provide ?
- hydrogen types: the number of signals shows the number of hydrogen environments
- integration: the peak area for each hydrogen type gives the number of H associated with each hydrogen type
- chemical shift: the position on x axis of each signal gives the electronic environment of each proton type. this includes hybridisation of attached carbon, presence of adjacent functional groups, etc
chemically equivalent hydrogens have:
- identical environments (interchangeable by bond rotation or a plane of symmetry)
- identical chemical shifts (share the same signal) number of hydrogen types = number of signals
how does shielding impact chemical shift?
the electrons around a nucleus create a magnetic field opposing the applied field. this reduces the apparent field, thereby shielding it from the applied magnetic field.
- shielded = more electron rich = upfield (lower chemical shift)
- deshielded = more electron poor = downfield (higher chemical shift)
how does electronegativity impact chemical shift?
electronegative atoms deshield and shift protons towards the left: higher radio frequency (more energy) needed for proton resonance
- increasing electronegativity means less electron density around H
- the electronegativity effect is roughly additive and depends on proximity (inductive effect diminshes with distance)
how does magnetic anisotropy affect chemical shift?
pi-electrons generate a local diamagnetic current that opposes the applied magnetic field (B0). This causes a shielding cone where nuclei inside the cone are shielded (smaller ppm) and nuclei outside the cone are deshielded (larger ppm)
- causes hydrogen atoms attached to pi systems to have a dramatic deshielding effect compared to alkane H’s
- adjacent pi systems also have a small deshielding effect
how does hydrogen bonding affect chemical shift?
hydrogen atoms on heteroatoms (OH and NH) are usually broad signals that have variable chemical shifts due to H bonding
define conformation
any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond
define a Newman projection
a way to view a molecule by looking along a carbon-carbon bond
define torsional or dihedral strain
repulsions between groups on adjacent atoms
why do alkane conformations exist?
bond rotations are possible between sp3 hybridised carbons (cylindrical symmetry of a sigma bond)
staggered conformation vs eclipsed conformation
staggered - bonds on adjacent C atoms are furthest apart (least torsional strain)
eclipsed conformation - bonds on adjacent C atoms are closest together (most torsional strain)
how does energy relate to stability of a molecule?
less stable -> higher energy
define steric strain
repulsive force that arises when atoms not bonded to each other are forced closer to one another (repulsion between electron clouds)
anti vs gauche conformation
anti: substituents are 180’ from one another -> no torsional strain + no steric strain
gauche: substituents are 60’ from one another -> no torsional train + YES steric strain
do cycloalkanes have more or less flexibility than open-chain alkanes?
cycloalkanes have less flexibility (less conformational freedom)
are rings larger than 3 atoms flat?
no; cyclic molecules adopt non-planar conformations to minimise angle and torsional strain
define angle strain
arise from bond angles that do not permit maximum orbital overlap between the atoms of a molecule (ideal=109.5’)
describe and draw cyclopropane
- 3-membered ring must be planar (flat) with bond angles of 60’
- requires that sp3-sp3 bonds are bent (reduced orbital overlap between carbons)
- all C-H bonds eclipsed (lots of torsional strain)
describe and draw cyclobutane
- if all 4 C’s were flat, there would be less angle strain than cyclopropane but more torsional strain
- torsional strain is alleviated in the butterfly conformation
describe and draw cyclopentane
- planar cyclopentane would have no angle strain but very high torsional strain
- non-planar conformation has very little ring strain and reduced torsional strain
- four carbon atoms are in a plane, fifth is above the plane: envelope conformation
describe and draw cyclohexane
- substituted cyclohexane rings occur widely in nature: free of angle strain and torsional strain
- tetrahedral angles between all carbons in the chair conformation
distinguish between axial and equatorial positions for chair conformations of cyclohexane
- axial are perpendicular to the ring plane
- equatorial are along ring plane
- every C atom has one axial and one equatorial H position
draw the axial and equatorial cyclohexane chair conformation groups
how do cyclohexane conformations have mobility?
chair conformations readily interconvert, resulting in exchange of axial and equatorial positions by a chair-flip
- all axial positions rotate to equatorial positions, while all equatorial positions rotate to axial
- each carbon has one up and one down position; up positions remain up after flip, while down positions remain down
why aren’t two conformations of any mono substituted cyclohexane equally stable?
due to 1,3-diaxial interactions: a type of steric strain between axial ring substituents
how do diaxial interactions affect the prevalence of a conformation?
a molecule will exist in the conformation where steric hindrance is minimised for longer
what impacts the magnitude of steric strain of one H-R 1,3 diaxial interaction?
- size and shape of the substituent affects the magnitude of steric strain.
- bulkier alkyl groups are more stable in the equatorial position
define stereoisomers
compounds in which the atoms are connected in the same order but differ in their spatial geometry
give an example of stereoisomers
- cycloalkanes with two or more substituents
- disubstituted cycloalkanes can exist as cis-trans stereoisomers
trans: two groups up/down on opposite face of ring
cis: two groups up/up or down/down on same face of ring
do a conformational analysis of cis-1,2-dimethylcyclohexane
do a conformational analysis of trans-1,2-dmethylcyclohexane
3 parts to a name, working backwards
- suffix (at the end): identifies the highest priority group
- root (middle): identifies the longest carbon chain and the natural of C-C bonds present
- prefix (at start): based on substituents, if any
- ol
alcohol
-amine
amine
-al
aldehyde
-one
ketone
-oic acid
carboxylic acid
-oate
ester
-oyl halide
acid halide
-oic anhydride
acid anhydride
-amide
amide
prefixes: methyl (Me)
prefixes: ethyl (Et)
prefixes: propyl (Pr)
prefixes: isopropyl (iPr)
prefixes: tert-butyl (tBu)
prefixes: cyclohexyl (Cy)
prefixes: phenyl (Ph)
prefixes: alcohol
hydroxy
prefixes: amines
amino
prefixes: ketones
oxo
