07.01 Why is Chirality Important?

having looked at the conformational

level of organic structure in detail

we're now going to transition to talking

about the configurational level which is

the province of a field called

stereochemistry the way we're going to

approach this is to first begin by

looking at a single molecule property

called chirality or handedness that

tells us that really that

stereochemistry is relevant to a

molecule chiral molecules are stereo

chemically active we might say then

we're going to transition into looking

at relationships between molecules in a

stereo chemical sense and look eight

types of isomerism where connectivity is

the same but configuration differs and

finally we're going to zoom in within a

particular molecule containing two

groups of the same type and ask about

their spatial relationship the essential

question there is are the groups in the

same spatial environment or different

spatial environments and if the answer

is different how are they different

let's jump right into talking about

chirality and stereo centers one of the

points that this slide makes is that

chirality is a property of any object

not just a molecule so we can think

about everyday objects in thinking about

chirality but this shows you the basic

idea of the molecular picture that a

right handed and left handed screw are

fundamentally different in the same way

a right-handed and a left-handed

molecule are different so we're going to

look at the criteria for a molecule to

be handed essentially how to determine

whether a molecule is handed and a

structural unit that is often associated

with chirality the stereocenter which is

a very important structural unit to

recognize for stereochemistry in general

anyway before all that I want to address

the question why is chirality important

one historical answer for this is that

ignoring chirality has potentially

deadly consequences in the late 1950s

the drug thalidomide was marketed to

pregnant women as a cure for morning

sickness the Lyne amide forms a mixture

of the two molecules you see here when

it's placed in the body and these two

molecules are isomers more specifically

we call them enantiomers one of these

enantiomers the one you see on the left

is the bioactive form that cures morning

sickness the other which is formed in

the body even if

we start with a pure sample of the

compound on the left causes birth

defects hopefully you can see the

problem here by ignoring the

stereochemical behavior of this molecule

inside the body we've given a drug to

pregnant women that cures one affliction

but causes one far worse the broader

point that this suggests is that

biochemistry is chiral chemistry by a

wide margin the vast majority of

important biochemical molecules are

chiral and so to properly understand the

behavior of these molecules it's not

sufficient just to know what's connected

to what to really appreciate how they

behave how they react and their

properties we also have to understand

something about the spatial orientations

of groups and the spatial relationships

between different molecules

this slide shows us examples of chiral

biochemical molecules both at the small

molecule level and at the polymeric or

macro molecular level here is the sugar

molecule glucose and here's glucose

wound up as an amylose polymer both of

these structures are chiral here's the

familiar double helix of DNA and if we

look at an individual monomer within the

DNA backbone this one in particular is

adenosine we see chirality as well and

finally here we have an amino acid this

is the amino acid tryptophan and the

amino acids are bound up into proteins

which are very large structures that are

asymmetric that are chiral and have a

handedness associated with them another

reason we studied stereochemistry is

just that it's intrinsically fascinating

stereochemistry is one of my favorite

areas of organic chemistry as a whole

and one of the reasons why is that we

can connect spatial properties of

molecules to spatial properties of

everyday objects and look at

interactions between everyday objects to

understand stereochemistry on a deep

level for example each and every one of

us is carrying around a pair of

enantiomers in our hands so even if you

don't know what enantiomers are yet take

my word for it that you're carrying

around a pair of enantiomers and your

hands have an enantiomeric relationship

the differences between enantiomers are

subtle in many ways they act identically

for example let's assume that you're

perfectly ambidextrous whether you use a

spoon with

left hand or your right hand is

irrelevant this is because the spoon

lacks a handedness we'll see what this

means a little bit later and for that

reason it doesn't really matter whether

you use a spoon with your left or right

hand both interactions feel the same

again assuming you're perfectly

ambidextrous the same is not true of an

object that has a handedness like this

coffee mug whether I hold this coffee

mug with my right or left hand matters

if my goal is to get liquid into my

mouth so the opening in the coffee cup

is here if I hold it with my right hand

and drink it looks like this but if I

hold the mug with my left hand and drink

now all of a sudden I have to do a goofy

motion like this there's something

fundamentally different about

interacting with this mug with my right

and left hands and that's because the

mug has a handedness will formalize this

and look at this in the molecular level

in the remainder of the videos in this