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Level 165

Structure & Stereochemistry of Alkanes


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acyclic
Not cyclic.
alkane *
A hydrocarbon having only single bonds; a saturated hydrocarbon; general formula: CnH2n?2.
alykl group
The group of atoms remaining after a hydrogen atom is removed from an alkane; an alkane-life substituent. Symbolized by R.
amyl
An older common name for pentyl.
angle strain or Baeyer strain
The strain associated with distorting bond angles to smaller (or larger) angles.
anti conformation
A conformation with a 180° dihedral angle between the largest groups. Usually the lowest-energy conformation.
aromatic hydrocarbon
A hydrocarbon having a benzene-like aromatic ring.
axial bond
One of six bonds (three up and three down) on the chair conformation of the cyclohexane ring that are parallel to the "axis" of the ring. The axial bonds are shown in red, and…
bridged bicyclic compound
A compound containing two rings joined at nonadjacent carbon atoms.
bridgehead carbons
The carbon atoms shared by two or more rings. Three chains of carbon atoms (bridges) connect the bridgeheads.
chair-chair interconversion (ring-flip)
The process of one chair conformation of a cyclohexane flipping into another one, with all the axial and equatorial positions reversed. The boat (or twist boat) conformation is an intermediate for the chair-chair interconversion.
cis-trans isomers (geometric isomers)
Stereoisomers that differ only with respect to their cis or trans arrangement on a ring or double bond.
combustion
Burning
common names
The names that have developed historically, generally with a specific name for each compound; also called trivial names.
conformational analysis
The study of the energetics of different conformations.
conformations and conformers
Structures that are related by rotations about single bonds. Strictly speaking, a conformer is a conformation that corresponds to a relative minimum in energy, usually a staggered conformation. In most cases, conformations and conformers intercon…
conformations of cyclohexanes
chair conformation: The most stable conformation of cyclohexane, with one part puckered upward and another part puckered downward.
constitutional isomers (structrual isomers)
Isomers whose atoms are connected differently; they differ in their bonding sequence.
cracking
Heating large alkanes to cleave them into smaller molecules.
cylic
Containing a ring of atoms.
cycloalkane *
An alkane containing a ring of carbon atoms; general formula: CnH2n.
degree of alkyl substitution
The number of alkyl groups bonded to a carbon atom in a compound or in an alkyl group.
1,3-diaxial interaction
The strong steric strain between two axial groups on cyclohexane carbons with one carbon between them.
dihedral angle (?) (see also conformations)
The angle between two specified groups in a Newman projection.
Eclipsed Conformation
All bonds are exactly lined up
equatorial bond
One of the six bonds (three down and three up) on the cyclohexane ring that are directed out toward the "equator" of the ring. The equatorial bonds are shown in green in the drawing at right.
fused ring system
A molecule in which two or more rings share two adjacent carbon atoms.
gauche conformation
A conformation with a 60° dihedral angle between the largest groups.
Halogenation
Addition of X and X
heat of combustion
The heat given off when a mole of a compound is burned with excess oxygen to give CO2 and H2O in a bomb calorimeter. A measure of the energy content of a molecule.
homologs
Two compounds that differ only by one or more --CH2-- groups.
hydrophilic
Attracted to water, soluble in water.
Hydrophobic
hates water
IUPAC names
The systematic names that follow the rules adopted by the International Union of Pure and Applied Chemistry.
kerosene
A thin, volatile oil distilled from petroleum, with a boiling range higher than that of gasoline and lower than that of diesel fuel. Kerosene was once used in lanterns and heaters, but now mos…
methane hydrate
An ice like substance consisting of individual methane molecules trapped inside cages of water molecules.
methine group *
The -CH- group.
methylene group
The -CH2- group.
Methyl Group
A functional group that consists of a carbon bonded to three hydrogens
n-alkane, normal alkane, or straight-chain alkane
An alkane with all its carbon atoms in a single chain, with no branching or alkyl substituents.
Newman projections
A way of drawing the conformations of a molecule by looking straight down the bond connecting two carbon atoms.
octane number
A rating of the antiknock properties of a gasoline blend. Its octane number is the percentage of isooctane (2,2,4-trimethylpentane) in an isooctane/heptane blend that begins to knock at the same compression ratio as the gasoline being tested.
paraffins
Another term for alkanes.
ring strain
The extra strain associated with the cyclic structure of a compound, as compared with a similar compound; composed of angle strain and torsional strain.
saturated
a solution that is in equilibrium with respect to a given dissolved substance. (maximum amount of solute under normal conditions)
sawhorse structures
A way of picturing conformations by looking down at an angle toward the carbon-carbon bond.
skew conformation
Any conformation that is not precisely staggered or eclipsed.
spirocyclic compounds
Bicyclic compounds in which the two rings share only one carbon atom.
Staggered Conformation
All bonds are distributed to be as far apart as possible
Steric strain
An increase in energy resulting when atoms are forced too close to one another
substituent
A side chain or appendage on the main chain.
systematic names
Same as IUPAC names, the names that follow the rules adopted by the International Union of Pure and Applied Chemistry.
torsional energy or conformational energy
The energy required to twist a bond into a specific conformation.
torsional strain
Decreased stability of a molecule associated with eclipsed bonds
totally eclipsed conformation
A conformation with a 0° dihedral angle between the largest groups. Usually the highest-energy conformation.
Alkane Formula
All C-C single bonds
Alkyl Groups
CH3 - methyl
Physical Properties
Almost totally insoluble in water due to low polarity and lack of hydrogen bonding.
Boiling Points of Alkanes
Boiling points increase with carbons
melting points
_______ indicate bond strength
Branche Alkanes
Lower boiling points with increased branching
C1-C2: gases (natural)
Major uses of Alkanes
Conformers of Alkanes
Structures resulting from the free rotation of a C-C single bond
Ethane Conformers
Staggered conformers has lowest energy
0
Discovered by Henry Cavendish in 1789
Butane Conformers C2-C3
Highest energy has methyl groups eclipsed
Higher Alkanes
Anti conformation is lowest in energy
amines
leave rarely/only under special conditions
Amides
Organic compounds whose molecules have a carbonyl nitrogen bond.
Chemistry of Alkanes
Saturated hydrocarbons are the most chemically inhert of all organic compounds.
Hydration
A process by which ionic solids dissolve in water: the positive ends of the water molecules are attracted to the negatively charged anions and the negatively charged ends are attracted to the positively charged cations
Oxidation (OIL)
[o] loss of hydrogen or electron, the gain of oxygen.
Reduction (RIG)
The loss of oxygen, the gain of hydrogen or an electron.
Esterfication
The formation of an ester from an alcohol and a carboxylic acid in the presence of an acid catalyst and heat. The reaction is a dehydration reaction because a water molecule is lost.
Hydrogenation
A reaction involving the addition of Hydrogen.
Lewis structure/Lewis electron dot formula:
Gives information about each atom's valence electrons.
Define valence:
The number of bonds an atom usually forms.
1
Valence of hydrogen:
4
Valence of carbon:
3
Valance of nitrogen:
2
Valence of oxygen:
1
Valance of fluorine:
2
Valence of sulfur:
5
Valence of phosphorus:
4
Valance of silicon:
1
Valence of chlorine:
Formal charge formula:
# valence electrons - # bonds - # nonbonding electrons
Dash formula:
Shows bonds between atoms but usually doesn't display lone pairs.
Condensed formula:
Shows neither the bond nor the 3D structure.
Bond-line formula:
Intersections, corners, and ends of a line represent carbon with the appropriate number of hydrogens.
Fischer projection:
Vertical lines are oriented into the page, and horizontal lines are oriented out of the page.
Newman projection:
A view straight down one of the s-bonds.
Dash-line-wedge formula:
Solid black wedges are out of the page, and dashed wedges are into the page.
s bond (sigma bond):
First type of covalent bond to be formed; strong.
p bond (pi bind):
Second type of covalent bond to be formed; from overlapping p orbitals; weaker.
A double bond has:
1 s-bond and 1 p-bond.
A triple bond has:
1 s-bond and 2 p-bonds.
Relationship between bond strength and bond length:
The stronger the bond, the shorter the length.
What causes rigidity in molecular structure?
Double or triple bond that cannot be rotated freely around.
What are hybrid orbitals?
An orbital formed by the mixing of two other orbitals, including sp, sp2, and sp3.
How to determine which hybrid orbital?
# sigma bonds + # lone pair electrons = sum of superscripts
What does the valence shell electron pair repulsion (VSEPR) theory state?
Electrons in an orbital seek to minimize their energy by moving as far away from other electron pairs as possible, minimizing the repulsive forces between them.
What can increase acidity?
If the conjugate base exhibits resonance.
Nucleophilic functional groups:
Have a partial negative charge; donate electrons; Lewis bases.
Electrophilic functional groups:
Have a partial positive charge; accept electrons; Lewis acids.
What is stereochemistry?
A 3D structure of a molecule.
What are isomers?
Unique moelleuse that share the same molecular formula.
Structural isomers, conformational isomers, and stereoisomers.
What are the major types of isomers?
What are structural isomers?
Compounds with the same molecular formula but different structural formulas.
What are conformational isomers?
Not true isomers; different spatial orientations of the same molecule.
Antistaggered, gauche, eclipsed, and fully eclipsed.
Different conformations from low energy to high?
What are stereoisomers?
They have the same molecular formula and same bond-to-bond connectivity.
Enantiomers and diastereoisomers.
What are the types of stereoisomers?
What are enantiomers?
non-superimposable mirror images
What are diastereoisomers?
Not mirror images of one another; have different chemical and physical characteristics.
A carbon is chiral when...
it is bonded to four different substituents.
What is absolute configuration?
When the lowest priority group is into page, the direction of decreasing priority goes clockwise (R) or counterclockwise (S).
When do two molecules have the same relative configuration?
If they differ by only one substituent and the other substituents are oriented identity around the chiral carbon.
What is specific rotation?
The molecular either rotates polarized light clockwise (+ or d) to counterclockwise (- or l) by a certain number of degrees.
What could a compound be optically inactive?
When it is either without chiral centers, or have identical mirror planes (mess compounds).
What is a racemic mixture?
An optically inactive mixture formed by adding equal amounts of each enantiomers.
2^n
equation for max number of stereoisomers
Configurational diastereoisomer, and cis-trans isomers.
What are the types of diastereoisomers?
Epimers, wh
What are some configuration diastereoisomers?
What is the anemic carbon?
The chiral carbon of an anomer.
What are cis/trans isomers?
Also called geometric isomers, these are due to (2 or more) substituents placement around a double bond. They either have the highest two priority molecules one the same side of the double bond (ci…
What is special about a cis isomer?
It may have steric hinderance, decreasing stability.
What is special about a trans isomer?
It may not have a dipole moment (if only disubstituted with he same molecule).