Chapter 5 Flashcards
What is the general formula of a noncyclic alkene?
What is the general formula of a cyclic alkene?
What is the general rules for hydrocarbon formulas?
CnH2n+2 minus two hydrogens for every p bond or ring in the molecule.
What is a compound’s degree of unsaturation?
The total number of p bonds and rings in a compound
in the general molecular formula, its CnH2n+2 minus two hydrogens for every degree of unsaturation
How many degrees of unsaturation does this compound have?
it has four fewer hydrogens than an acyclic alkane with eight carbon (CnH2n+2=C8H18)
so, 2 degrees of saturation
Why do the names ethylene and propylene exist but not butylene?
Because butylene could mean either 1-butene or 2-butene and there can’t be two compounds with the same name.
How do you name a compound with more than one double bond?
the “ne” ending of the corresponding alkane is replaced with “diene,” “triene,” “tetraene,” and so on, depending on the number of double bonds in the parent hydrocarbon.
What happens when there are both an unsaturation and a substituent?
the unsaturation suffix gets the lowest possible number.
How do you number a ring that has a double bond and a substituent?
the double bond is always between carbons 1 and 2.
To assign numbers to any substituents, count around the ring in the direction that puts the lowest number into the name.
What is a vinylic carbon?
The sp2 carbons of an alkene
What is a allylic carbon?
An sp3 carbon that is adjacent to a vinylic carbon
What is a vinylic hydrogen?
A hydrogen bonded to a vinylic carbon
What is a allylic hydrogen?
a hydrogen bonded to an allylic carbon
What is a vinyl group?
the smallest possible group that contains a vinylic carbon
What is an allyl group?
the smallest possible group that contains an allylic carbon
What if “vinyl” or “allyl” is used in a name?
the substituent must be attached to the vinylic or allylic carbon, respectively.
What is the yellow group called? What is the full name of this compound?
What is the yellow group called? What is the full name of this compound?
What is the yellow group called? What is the full name of this compound?
What is the yellow group called? What is the full name of this compound?
What is a double bond made of?
Each double bonded carbon of an alkene has three sp2 orbitals. Each of these orbitals overlaps an orbital of another atom to form a s bond, one of which is one of the bonds in the double bond.
The other bond of the double bond is a p bond formed from side-to-side overlap of the remaining p orbital on each of the sp2 carbons.
all carbons lie in the same plane to achieve maximum overlap as the p orbitals can be parallel to each other
How can we predict the reaction of a particular organic compound?
organic compounds can be divided into families, and all the members of a family react in the same way.
What determines the family of to which an organic compound belongs to?
Its functional group
ex: functional group of an alkene: the carbon–carbon double bond. All compounds with a carbon–carbon double bond react in the same way (ethene and cholesterol)
Is unsaturation considered as functional groups?
Yes
How many groups can the compound families be separated in?
What is in common amongst families of the same group?
4
they react the similar ways
What is another name for “degree of unsaturation”?
IHD: index of hydrogen deficiency
What is another name for “common names”?
Functional class names (FC)
What is another name for “systematic names”?
Substitutive names (SN)
What is group 1 about?
the reactions of alkenes and alkynes
What are organic chemistry reactions all about?
the interaction between electron-rich species and electron-deficient species
What are electron-deficient species?
electrophiles or Lewis acids
they look for electrons
How do you recognize an electrophile?
it has either a positive charge, a partial positive charge, or an incomplete octet that can accept electrons.
What are electron-rich species?
nuclophiles or Lewis bases
How can you recognize nuclophiles?
has a pair of electrons it can share.
When is a lewis base called a “base” and when is it called a “nucleophile”? (for the purpose of the manual)
base when a Lewis base reacts with a proton
nucleophile when it reacts with something other than a proton
Why is it important to distinguish between “base” and “nuclophile” when it comes to they reaction with a proton or something other than a proton?
because base strength is a thermodynamic property and nucleophile strength is a kinetic property
(they determine different things)
Why are alkynes nucleophiles?
The clouds of electrons surrounding the sigma bond makes an alkyne an electron-rich molecule. They are therefore nucleophiles that react with electrophiles.
What makes H-Br or H2O electrophiles?
because of the dipole moment created in the two polar molecules (the partial positive charges of H are the electrophiles)
Why are alkenes nuclophiles?
because of the electron rich area below and above the sigma bond
Which bond is broken in alkenes when it undergoes a reaction? Why?
the pi bond because it is weaker than the sigma bond
What would happen if we react a reagent such as hydrogen bromide with an alkene?
What would be the product of the reaction?
Step 1: the alkene (a nucleophile) will react with the partially positively charged hydrogen (an electrophile) of hydrogen bromide;
the product of the reaction will be a carbocation.
Step 2: the positively charged carbocation (an electrophile) will react with the negatively charged bromide ion (a nucleophile)
this forms an alkyl halide.
What is the step-by-step description of a process by which reactants change into products such as this one is called?
it is the mechanism of the reaction
How are curved arrows used to help us understand a mechanism?
For example in a reaction of an alkene with hydrogen bromide (H-Br), in the first step, what happens for the hydrogen to leave its bond with bromine and share it with alkene? What happens to the electrons previously shared between the hydrogen and bromine?
As the pi bond in the alkene approaches the hydrogen, the H-Br bond breaks, the pi bond shares and electron with hydrogen and bromine keeps the bonding electrons
What is the electron flow?
electrons always flow from negative to positive charge
Do curved arrows go along or against electron flow?
along
Can curved arrows also indicate movements of atoms? Movement of a proton for example?
No
Is this a good representation of electron transfer?
No, a curved arrow starts at an electron source; it does not start at an atom.
In the following example, the arrow starts at the electron-rich p bond, not at a carbon atom:
Why is this wrong?
What is a reaction coordinate diagram?
it shows the energy changes that take place in each of the steps of a mechanism of reaction.
total energy of all species is plotted against the progress of the reaction.
Because a reaction progresses from left to right as written in a chemical equation, the energy of the reactants is plotted on the left-hand side of the x-axis and the energy of the products is plotted on the right-hand side.
Is stability associated with low or high energy?
low
How is the “maximum energy state” called in a reaction coordinate diagram?
transition state
What is the structure of the transition state?
between the structure of the reactants and the structure of the products
What are the dash lines in the structure of the transition state?
to show partially broken or partially formed bonds.
What happens if the transition state is too high?
the reactants will not be able to be converted to products, so no reaction will take place.
What is the difference between the kinetics and thermodynamics of a reaction?
thermodynamics: the relative amounts of reactants (Y) and products (Z) present when the reaction has reached equilibrium
kinetics: how fast Y is converted to Z
On what does relative concentrations of products and reactants at equilibrium depend on?
their relative stabilities
Why is the strength of an acid is determined by the stability of its conjugate base?
as the base becomes more stable, the equilibrium constant (Ka) for its formation becomes larger—and the larger the Ka, the stronger the acid.
What is an exergonic reaction?
When gibs free energy change (delta G) is negative
What is an endergonic reaction?
When gibs free energy change (delta G) is positive
Are products or reactants more stable when delta G is negative?
products
Are products or reactants more stable when delta G is positive?
reactants
What is enthalpy
the heat given off or the heat consumed during the course of a reaction.
When is heat released and consumed in reactions?
Released when bonds form
Consumed when bonds are broken
How can we calculate the enthalpy (delta H) of a reaction?
In an exothermic reaction, are bonds formed weaker or stronger than the bonds broken?
stronger
What is Le Châtelier’s principle?
If an equilibrium is disturbed, the system will adjust to offset the disturbance.
In other words, if the concentration of C or D is decreased, then A and B will react to form more C and D in order to maintain the value of the equilibrium constant.
What is a metabolic pathway?
series of sequential reactions
What can be done for endergonic reactions to produce more product? What principle is applied?
Some of the reactions in a metabolic pathway are endergonic and, therefore, produce very little product.
However, the amount of product produced is increased if the endergonic reaction is followed by a highly exergonic reaction
Le Châtelier’s principle.
What are two sequential reactions (an endergonic reaction followed by an exergonic reaction) called?
coupled reactions
How are coupled reactions important for metabolic pathways?
they are the thermodynamic basis for how metabolic pathways are regulated
What are catalytic hydrogenations?
What is necessary for H2 to add to the double bond of an alkene? How is this addition of hydrogen called?
a metal catalyst
hydrogenation
What is produced by the hydrogenation of an alkene?
an alkane
What is the most common metal catalyst? How is it abbreviated?
palladium
Pd/C (palladium on carbon)
What does the metal catalyst do in a hydrogenation reaction?
to weaken the very strong H ¬ H bond.
What is a reduction reaction? Give an example of reduction reaction.
A reduction reaction increases the number of C¬H bonds and/or decreases the number of C¬O, C¬N, or C ¬ X bonds (X is a halogen).
catalytic hydrogenation
Explain the catalytic hydrogenation of an alkene into an alkane.
hydrogen is adsorbed on the surface of the metal and the alkene interacts with the metal by overlapping its p orbitals with the vacant orbitals of the metal.
All the bond-breaking and bond- forming events occur on the surface of the metal.
As the alkane product forms, it diffuses away from the metal surface
What is a simple way to explain catalytic hydrogenation?
both the H¬H bond of H2 and the p bond of the alkene break, and then the resulting hydrogen radicals add to the resulting carbon radicals.
How do you chose the reactant for a synthesis?
choose an alkene with the same number of carbons as the desired product and attached in the same way as those in the desired product.
ex: to make methylcyclohexane, these alkenes can be used;
What is heat of hydrogenation and is it positive or negative?
The heat released in a hydrogenation reaction
Hydrogenation is exothermic so delta H is negative, but the heat of hydrogenation is the same value as delta H, but without the negative sign (so positive)
Would an stable alkene have more or less heat of hydrogenation? Why?
less, because the reactant would have less energy to begin with
What determines the stability of alkenes?
stability of an alkene increases as the number of alkyl groups bonded to the sp2 carbons increases.
Is cis or trans isomer of an alkene more stable for the same number of attached alkyl groups on the sp2 carbons? Why?
trans isomer
has a lower heat of hydrogenation due to large groups being farther apart, making it more stable than the cis isomer
Why are cis isomers less stable than trans isomers?
electron clouds of large groups interfere with each other when on the same side, causing steric strain
How do the heat of hydrogenation of these two molecules compare?
They are similar since the total of alkyl groups bonded to sp2 carbons are the same
What exactly is free energy of activation?
the energy barrier of a reaction
many factors can affect this energy barrier
its the difference between the free energy of the transition state and the free energy of the reactants
What happens when the height of the energy barrier is decreased?
the rate of the reaction increases
How can you decrease the height of the energy barrier?
anything that makes the reactants less stable
or
makes the transition state more stable
