Ch 2 - Molecular Representations Flashcards
4 types for small molecules
- Lewis structure
- partially condensed structure
- condensed structure
- molecular formula
Lewis Structure
- all atoms and bonds are explicitly drawn
- only practical for very small molecules(large is hard to draw)
partially condensed structure
- drawing style in which CH bonds are not drawn explicitly but all other bonds are
- only practical for small molecules
condensed structures
- drawing style in which none of the bonds are drawn
- groups of atoms are clustered together when possible
- (CH3)2CHOH
- only practical for small molecules
molecular formulas do not
provide enough information
Bond-Line Structures
- most common drawing style employed by organic chemist
- all carbon atoms and most hydrogen atoms are implied but not explicitly drawn
- each corner or end point is a carbon atom
- triple bonds are drawn in a linear format(sp hybridized carbon) and will have linear geometry
- each carbon in a bond line is assumed to have enough Hydrogen to complete 4 bonds so the hydrogens are not drawn
How to drawn Bond-Line structures
- carbon atoms in a straight chain should be drawn in a zigzag format
- when drawing double bonds, draw all bonds as far apart as possible
- when drawing single bonds, the direction in which the bonds are drawn is irrelevant
- all heteroatoms(atoms other than C and H) must be drawn, and any hydrogen atoms attached to a heteroatom must be drawn
- never draw a carbon atom with more than four bonds. Carbon only has four orbitals in its valence shell and therefore carbon can only form four bonds
converting Lewis structure to Bond-line
- delete hydrogen atoms(except those connected to heteroatoms)
- draw in zigzag formal keeping triple bonds linear
- delete carbon atoms
functional group
a characteristic group of atoms/bonds that possess a predictable chemical behavior
C=C is a functional group
Alkene
- typically react with molecular hydrogen(H2) in the presence of a catalyst
the chemistry of every organic compound is determined by
the functional groups present in the compound
R-X
- X = Cl,Br,or I
- alkyl halide
R-(R-)C=C-R(-R)
alkene
R-C-=C-R
Alkyne
R-OH
Alcohol
R-O-R
Ether
R-SH
Thiol
R-S-R
Sulfide
Benzene ring of 6 carbon(double bond every other)
Aromatic(or arene)
R-C(=O)-R
Ketone
R-C(=O)-H
Aldehyde
R-C(=O)-O-H
Carboxylic Acid
R-C(=O)-X
Acyl halide
R-C(=O)-O-C(=O)-R
Anhydride
R-C(=O)-N(-R)-R
Amide
R-C(=O)-O-R
Ester
R-N(-R)-R
Amine
a carbon atom will generally have four bonds only when it does not have a formal charge
- either a + or – charge on carbon will result in 3 bonds
a + on carbon means 3 bonds total so H will
fill in missing spots(0,1,2)
a – on carbon will still form 3 bonds total and have
1 extra lone pair(5 valence electrons total)
C+ the 4th orbital is _____ and C- the fourth orbital holds ______.
empty
a lone pair of electrons
three bond categories
- covalent(sharing)
- polar covalent(uneven sharing)
- ionic(electron transfer)
the bond type is based on
the electronegativity(measure of an atoms ability to attract electrons) difference of two atoms
General guideline for electronegativity(not definitive)
- 0.0-0.5 is covalent(C-C)
- 0.5-1.7 is polar covalent bond(C-O)
- 1.7+ is ionic(NaOH)
induction
the withdrawal of electron density that occurs when a bond is shared by to atoms of differing electronegativity
to determine the formal charge on an atom we must know how many lone pairs it has
- or to determine lone pairs we must know the formal charge
a bond line structure will only be clear if it contains either all of the lone pairs or all of the formal charges
- conventionally formal charges will be drawn(less of them to draw usually)
formal charges must always be drawn, unlike
lone pairs which can be omitted
2 steps for determining the number of lone pairs(per section 1.4)
- determine the appropriate number of valence electrons for the atom
- O should have 6 valence electrons
- determine if the atom actually exhibits the appropriate number of electrons
- C-O means O exhibits 7 valence electrons which means negative and is must ave 3 lone pairs
Oxygen atom formal charge patterns
- negative charge corresponds with one and three lone pairs
- absence of charge corresponds with 2 bonds and 2 lone pairs
- positive charge corresponds with three bonds and one lone pair
Nitrogen atom formal charge patterns
- negative charge corresponds with two bonds and two lone pairs
- absence of charge corresponds with three bonds and one lone pair
- positive charge corresponds with four bonds and no lone pairs
wedges and dashes
- wedge is coming out of the page(toward you)
- dash is going behind the page(away from you)
resonance
draw reciprocal bond line structures and think of them as being melded together
resonance structure
drawing showing how positive charge is spread over two locations at the same time
resonance hybrid
the resonance structure is placed in brackets with between the two parts indicating resonance
- the entity is not flipping back between the two drawn figures
no single drawing will be adequate to describe resonance
- there must be a melding of images in your mind
resonance does not describe something that is happening but
is a way to deal with the inadequacy of bond-line structures
delocalization
the spreading of positive or negative charge of an over two locations
resonance stabilization
the delocalization of either positive or negative charge to stabilize a molecule
- plays a major role in the outcome of many reactions
curved arrows
tool required to draw resonance structures properly
- always a tail and head to the arrow - do not represent the motion of electrons - they treat electrons as if they were moving, even though not usually moving
the arrow must be drawn precisely in the right location
- the tail is where the electrons are coming from
- head is where the electrons are going(not actually but as a representation)
never draw curved arrows:
- avoid breaking a single bond
- never place the tail on a single bond
- never exceed an octet for second-row elements.
- CNOF can only have 4. Never 5 or 6
when multiple arrows are used it is acceptable for one to violate the rules i
f the end result does not
2 steps to formal charge in resonance structures
- carefully read what the curved arrows indicate
- assign formal charges
the total charge of both resonance structures MUST
be the same
five patterns in resonance patterns
- allylic lone pair
- allylic positive charge
- lone pair adjacent to a positive charge
- pie bond between two atoms of differing electronegativity
- conjugated pie bonds in a ring
allylic lone pair
- vinylic position – C=C the two carbons are in these positions
- allylic position – atoms connected directly to the vinylic positions
- requires two curved arrows for resonance
- the first goes from the lone pair to forma pie bond
- the second goes from the pie bond to form a lone pair
- when the atom with the lone pair has a negative charge then the negative charge is transferred to the atom that ultimately receives the lone pair
- if the atom with the lone pair does not have a negative charge then it will incur a positive charge
vinylic position
C=C the two carbons are in these positions
allylic position
atoms connected directly to the vinylic positions
allylic positive charge
- only one curved arrow will be used
- conjugated – when there are two pie bonds separated by exactly one sigma bond
- when there is a positive charge we move the positive from one end of the other end of the system
- never place the tails of an arrow on a + sign
- put them on the pie bond
lone pair adjacent to a positive charge
- if the atom with lone pairs has a negative charge then the + and – cancel
- not true for Nitro groups(NO2 where N is + and O is -)because it will violate the octet rule for N
- requires two curved arrows resulting in – switching from one O to the other and N staying +- if the atom with lone pairs is neutral then it will become + by the end
- one curved arrow
pie bond between two atoms of differing electronegativity
- when the pie bond in a double bond is split then one atom gets the – while the other gets +
- one curved arrow
conjugated pie bonds in a ring
- in a closed ring if one pie bond in a double ring is pushed then all are
- rotates an Arene one bond over for each bond- three curved arrows
3 factors to resonance importance
- minimizes charge
- electronegative atoms like N,O,Cl can bear a positive charge but only if they possess an octet of electrons
- avoid drawing a resonance structure in which two carbon atoms bear opposite charge
3 factors to resonance importance
minimizes charge
- the best resonance structure is one without charge
- structures with more than 2 charges should be avoided
- the Nitro(NO2) group always has a minimum of two charges so they are considered 0(a 4 charge would actually be considered 2 and thus significant)
3 factors to resonance importance
electronegative atoms like N,O,Cl can bear a positive charge but only if they possess an octet of electrons
- O can have 3 bonds + a lone pair which = 5 electrons(1 less than normal) and thus positive
- the most significant factor in resonance structures is whichever structure gives all atoms a complete octet
3 factors to resonance importance
avoid drawing a resonance structure in which two carbon atoms bear opposite charge
resonance structures in which there are C= and C- are insignificant
delocalized lone pair
a lone pair that is allylic to a pie bond and will participate in resonance
when an atom possesses a delocalized lone pair the geometry of the atom is
affected by the lone pair
An amide is expected to be an sp^3 hybrid but is actually sp^2 with the lone pair creating the pie bond in C=N
localized lone pair
a lone pair that does not participate in resonance(not allylic to a pie bond)
whenever an atom possesses both a pie bond and a lone pair, they will not both participate in resonance
- in general the pie bond will and the lone pair will not
- the pie bond takes the p orbital disallowing the lone pair to fill the same space
