Ch. 4 - Alkanes Flashcards
What type of interaction(s) destabilize a cyclohexane chair conformation that has a bulky substituent in an axial position?
steric interactions, 1,3-diaxial interactions
True or false: Alkanes undergo few reactions since they are the only family of organic molecules that do not contain a functional group.
True
What is an alkane?
aliphatic hydrocarbon with only C-C and C-H bonds (sigma bonds)
What are acyclic alkanes and what is their formula?
saturated hydrocarbons, chains; CnH2n+2
What is a saturated hydrocarbon?
has max number of hydrogens (check!!)
How are degrees of unsaturation calculated?
degrees of unsaturation = (2C + 2 + N – H – X) / 2
What are cycloalkanes and what is their formula?
rings; CnH2n
What are isomers?
2 different compounds with same molecular formula
What are constitutional isomers?
differ in connectivity
What are stereoisomers?
differ in space
What is a normal alkane?
a straight-chain (n-alkane)
Does bending of a molecule chain change its identity?
no
How does the number of constitutional isomers change as the number of carbons changes?
As # of carbons increase, # const. isomers increase
What is a methylene group?
CH2; creates every consecutive acyclic alkane
What is a homologous series?
group of compounds that differ only by a CH2 group
1 carbon
methane
2 carbons
ethane
3 carbons
propane
4 carbons
butane
5 carbons
pentane
6 carbons
hexane
7 carbons
heptane
8 carbons
octane
9 carbons
nonane
10 carbons
decane
11 carbons
unedecane
12 carbons
dodecane
13 carbons
tridecane
14 carbons
tetradecane
15 carbons
pentadecane
20 carbons
icosane
What is the systematic name?
IUPAC name, general structure
What is the generic name?
official and approved
What is the trade name?
assigned by a company
What makes up an alkane name?
prefix + parent + suffix
What does the parent name respresent?
number of carbons in the longest continuous chain
What does the suffix represent?
functional group (alkane)
What does the prefix represent?
identity, location, number of substituents
What are alkyl groups? How are they formed?
carbon substituents bonded to long carbon chain
formed by removing a hydrogen from alkane
How are alkyl groups named?
change -ane to -yl
How many alkyl groups are there for propanes?
2; propyl and isopropyl
How many alkyl groups are there for butanes?
4; butyl, sec-butyl, isobutyl, tert-butyl
What is 2-methylpropane?
forms butyl substituents (isobutyl or tert-butyl)
What is the parent of the chain?
longest continuous carbon chain
What do you do if there are two carbon chains of equal length (parent)?
pick chain with more substituents
What are the steps for naming IUPAC compounds (simple)?
1) Find parent chain and add suffix -ane
2) Number atoms in carbon chain giving first substituent the lowest number
3) Name and number the substituents as (-yl) groups, denote di, tri, tetra, etc if there is more than one of that type of substituent
4) Combine the substituent names and numbers based on alphabetical order (ignoring prefixes except for iso) _ parent + suffix
(numbering carbon atoms) If the first substituent is the same number from both sides of chain, what should be done?
number the chain to give the second substituent the lower number
When substituents are numbered the same from either end, how should they be numbered?
number the chain so the lowest number is given alphabetically
T/F: A carbon can belong to both a parent chain and a substituent.
False. It can only belong to one or the other
How are the substituents names placed in the fully combined name?
alphabetically, ignoring all prefixes except for -iso
What is the structure for naming a cycloalkane?
prefix + cyclo + parent + suffix
T/F: No number is needed to indicate a single substituent on a cycloalkane.
True
How should substituents be numbered in a ring?
assign lower numbers to substituents alphabetically
What do you do if an alkane has both a chain and a ring?
if the number of carbons in the ring >/= to the number of carbons in longest chain, it is a cycloalkane
What is natural gas composed of?
methane
Do alkanes have nonpolar bonds?
yes, C-C and C-H are nonpolar
What types of intermolecular forces do alkanes have?
van der waals (weak) only
Are alkanes water soluble?
no, nonpolar
What does is the trend for the boiling/melting point of alkanes?
as the number of carbons increases, so does the surface area and so does the BP and MP (IMFs increase)
How does branching affect the BP of isomers?
decreases with increased branching (decreases surface area)
How does the symmetry affect the melting point of isomers?
symmetry increases melting point
What are alkanes soluble in? Water or organic solvents?
organic solvents
What is stereochemistry?
3D structure of molecules
What are conformations?
different arrangements of atoms that are interconverted by rotation around single sigma bonds (either eclipsed or staggered)
still the same compound!!
What is the eclipsed conformation?
C-H bonds of both carbons are directly aligned (on top of each other looking down)
What is the staggered conformation?
C-H bonds on one carbon bisect the H-C-H bond angle on adjacent carbon (don’t overlap when looking down)
What is the degree of rotation between the staggered and eclipsed conformations?
60º
What is a dihedral angle?
angle that separates a bond on one atoms from a bond on another (0º or 60º)
What is a Newman projection?
the graphic that shows the three groups bonded to each carbon in a particular C-C bon and their dihedral angles
T/F: Staggered and eclipsed conformations interconvert at room temp but are not equally stable.
true
Which is the more stable conformation, staggered or eclipsed?
staggered
Why is the staggered conformation more stable than eclipsed?
less e- – e- repulsion
How much does each C-H bond eclipsed increase the energy by?
4 kJ/mol
What is the energy difference between eclipsed and staggered conformations?
12 kJ/mol
What is torsional energy?
energy difference between staggered and eclipsed conformations
What is torsional strain?
increased in E caused by eclipsing interactions within a molecule
Is staggered anti or staggered gauche higher in energy?
staggered gauche, electron density is closer together
What do relative energies of conformations depend on?
steric strain
What is steric strain?
increase in energy that results from atoms being forced too close to one another
How do larger groups that are closer in eclipsed conformation affect steric strain?
increase eclipsed energy
What is the barrier to rotation?
energy difference between the lowest and highest E conformations
Does the zig zag of alkanes assume eclipsed or staggered conformation?
staggered
What is angle strain?
increase in E when a tetrahedral bond angle deviates from optimum angle (109.5º); only affects cycloalkanes
What is the Baeyer strain theory?
rings with bond angles different that tetrahedral bond angles are strained and reactive; therefore carbon rings cannot be flat
What is cycloalkane puckering?
contracted cycloalkanes to reduce angle strain (smaller the angle, less stable)
What is the most common ring in nature?
cyclohexane
In reality, what is the conformation of cyclohexanes?
chair conformation
What is the chair conformation?
puckered cyclohexane conformation that eliminate angle strain and torsional strain and is therefore more stable
all hydrogens are staggered as well
How many up Cs and down Cs are there in puckered ring?
3 ups, 3 downs
What are axial groups?
located above or below the ring in perpendicular axis
up carbon, up axial; down carbon, down axial
What are equatorial groups?
located in the plane of the ring (up carbon, down equatorial; down carbon, up equatorial)
T/F: cyclohexane does not remain in a single conformation
true
What is ring flipping?
conformational change of chair to boat to chair where up carbons become down and down carbons become up (axials become equatorial and equatorials become axial)
How much more stable are chair forms compared to boat, why?
chair forms are 30 kJ/mol more stable
boat has torsional strain because carbons are eclipsed, steric strain because 2 hydrogens are at either end of the boat
What are flagpole carbons?
up carbons on either end of boat conformation; forced close together causing steric strain
Does the equatorial or axial position better accommodate large substituents?
equatorial position, has more space
T/F: substituents move above or below the ring when a chair conformation intercoverts
false, substituents always stay above or below the ring and never switch
How does the presence of a large substituent affect the percentages of certain chair conformations?
the % of the conformation with large sub on equatorial will be larger because it is more stable
How many chairs does a cis/trans cyclohexane have?
2 chairs
How are cis and trans chair conformations drawn?
cis: same subs on same side (up/down) of ring
trans: same subs on different side of ring
What are the two types of chair conformations that can form for a trans hexane? Which is more stable?
diaxial or diequatorial
diequatorial is more stable
What are two types of chair conformations that can form for cis hexane? Which is more stable?
one of axial, one on equatorial and vice versa
equally stable, 50%
