Introduction to Organic Compounds Flashcards
(124 cards)
1
Q
define organic chemistry
A
the study of compounds containing carbon
2
Q
why is carbon the most abundant element in living things?
A
- 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
3
Q
define hydrocarbons
A
organic compounds made up of only carbon and hydrogen
4
Q
alkanes
A
- only C-C bonds
- general formula CnH2n+2
5
Q
alkenes
A
- contain a C=C bond
6
Q
alkynes
A
- contain a C≡C bond
7
Q
state the four kinds of orbitals for electrons, arranged from lowest energy to highest energy
A
s, p, d, and f
8
Q
where is the probability of finding an electron highest?
A
near the nucleus; the probability decreases as the distance from the nucleus increases
9
Q
shape of an s orbital
A
spherical
10
Q
shape of p orbitals
A
- 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)
11
Q
rules for filling the ground-state electron configuration (lowest energy arrangement)
A
- 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
12
Q
why do valence electrons participate in bonding?
A
as they are less stable
13
Q
why do bonds form?
A
because molecules have a lower energy than individual atoms
14
Q
ionic bonding
A
- electrostatic attraction between oppositely charged ions
- bond forms from electron transfer
15
Q
covalent bonding
A
- sharing of electrons between atoms
- each bonding atom usually contributes one electron to the new bond (opposite spin)
16
Q
sigma bond
A
- type of covalent bond
- head on orbital overlap
- s-s, s-p, p-p overlap
17
Q
pi bond
A
- type of covalent bond
- side-to-side orbital overlap
- p-p orbitals
18
Q
electronegativity
A
the ability of an atom to pull electrons toward itself through a bond
19
Q
give the formula for Formal charge
A
FC = (group #) - (# of bonds) - (# non-bonded electrons)
20
Q
define VSEPR
A
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
21
Q
3 possible structural geometries of carbon
A
- tetrahedral
- trigonal planar
- linear
22
Q
tetrahedral
A
- four groups around carbon are equally distributed in a pyramid arrangement
- bond angle of about 109’ between atoms
23
Q
trigonal planar
A
- three groups around carbon are equally distributed in the same plane (flat)
- bond angle of about 120’ between atoms
24
Q
linear
A
- two groups around carbon are equally distributed in a line
- bond angle of 180’ between atoms
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Valence bond theory
covalent bond involves the sharing of two spin-pasted electrons through the overlap of atomic orbitals
better orbital overlap = stronger bond
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define hybrid orbitals
atomic orbitals that mix to give the observed geometry of atoms
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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
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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'
29
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'
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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
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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)
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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
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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
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sp3 hybridisation of nitrogen
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sp3 hybridisation of oxygen
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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
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constitutional isomers
molecules with the same molecular formula but different bonding
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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
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define a delocalised pi bond
sharing of 2 electrons among more than 2 atoms
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why does delocalisation take place?
sharing of electrons among many atoms (delocalisation) stabilises molecules
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three characteristics of resonance forms
- identical placement of atoms
- different placement of electrons (lone pairs and pi electrons)
- a double-headed arrow separating them
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what is a resonance hybrid?
the actual structure of the molecule, a blend of all the resonance forms that show the delocalised electrons
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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
44
give an example of pi bond with atoms of different electronegativities (resonance forms)
45
give an example of a pi bond directly beside paired electrons (resonance forms)
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give an example of a pi bond being directly beside an atom with an incomplete octet
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give an example of pi bonds being directly beside other pi bonds
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give an example of an atom with an incomplete octet next to an atom with a lone pair of electrons
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structures cccan have more than 2 resonance forms if
more than 3 atoms in sequence have p orbitals
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do resonance structures contribute equally to the resonance hybrid?
no; forms with favourable electron distribution are stronger contributors
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define a functional group
an atom or specific groups of atoms which give molecules distinct reactivity and properties
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hydrocarbons
- alkane
- alkene
- alkyne
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organohalide
X = F, Cl, Br, I
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amine
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alcohol
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ether
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nitrile
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types of carbonyl groups
- ketone
- aldehyde
- carboxylic acid
- ester
- amide
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define the inductive effect
the distribution of electron density in a bond due to differences of electronegativity
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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
61
define electrostatic interactions
when oppositely charged molecules attract
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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
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define hydrogen bonding
the attractive force between a N or O lone pair and a H in OH or NH
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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
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boiling and melting points are a reflection of
relative strength of intermolecular forces
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as alkane size increases, mp and bp
increase due to increasing dispersion forces
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polar groups impart higher mp/bp values due to
increased dipole-dipole interactions (strong), hydrogen bonding (stronger), or electrostatic effects (strongest)
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state the three types of solvent
- polar protic solvents
- polar aprotic solvents
- non polar solvents
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polar protic solvents
H-bond donors, eg OH, NH, very polar
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polar aprotic solvents
strong dipoles (polar), most are H-bond acceptors
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non polar solvents
mainly non polar bonds or no significant net dipole
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what type of solvent is ether
non polar
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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)
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'many organic molecules are polar or weakly polar and dissolve in
polar solvents
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highest quality resonance forms have the following characteristics
1. most atoms with full octets
2. fewest number of formal charges
3. if formal charges exist, the negative charges are on the most electronegative atoms while positive charges on the most electropositive atoms
4. like charges separated by the maximum distance possible, while opposing charges as close together as possible
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acyl group
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acetyl
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formyl
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benzoyl
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3 parts to a name, working backwards
1. suffix (at the end): identifies the highest priority group
2. root (middle): identifies the longest carbon chain and the natural of C-C bonds present
3. prefix (at start): based on substituents, if any
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- ol
alcohol
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-amine
amine
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-al
aldehyde
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-one
ketone
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-oic acid
carboxylic acid
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-oate
ester
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-oyl halide
acid halide
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-oic anhydride
acid anhydride
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-amide
amide
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prefixes: methyl (Me)
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prefixes: ethyl (Et)
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prefixes: propyl (Pr)
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prefixes: isopropyl (iPr)
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prefixes: tert-butyl (tBu)
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prefixes: cyclohexyl (Cy)
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prefixes: phenyl (Ph)
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prefixes: alcohol
hydroxy
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prefixes: amines
amino
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prefixes: ketones
oxo
100
define conformation
any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond
101
define a Newman projection
a way to view a molecule by looking along a carbon-carbon bond
102
define torsional or dihedral strain
repulsions between groups on adjacent atoms
103
why do alkane conformations exist?
bond rotations are possible between sp3 hybridised carbons (cylindrical symmetry of a sigma bond)
104
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)
105
how does energy relate to stability of a molecule?
less stable -> higher energy
106
define steric strain
repulsive force that arises when atoms not bonded to each other are forced closer to one another (repulsion between electron clouds)
107
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
108
do cycloalkanes have more or less flexibility than open-chain alkanes?
cycloalkanes have less flexibility (less conformational freedom)
109
are rings larger than 3 atoms flat?
no; cyclic molecules adopt non-planar conformations to minimise angle and torsional strain
110
define angle strain
arise from bond angles that do not permit maximum orbital overlap between the atoms of a molecule (ideal=109.5')
111
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)
112
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
113
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
114
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
115
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
116
draw the axial and equatorial cyclohexane chair conformation groups
117
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
118
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
119
how do diaxial interactions affect the prevalence of a conformation?
a molecule will exist in the conformation where steric hindrance is minimised for longer
120
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
121
define stereoisomers
compounds in which the atoms are connected in the same order but differ in their spatial geometry
122
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
123
do a conformational analysis of cis-1,2-dimethylcyclohexane
124
do a conformational analysis of trans-1,2-dmethylcyclohexane
