Kinetic vs Thermodynamic Product - 1,2 vs 1,4 Addition of HBr to 1,3- Butadiene

Summary

This educational video explores the addition reactions of dienes with hydrobromic acid (HBr) under varying conditions. It explains the concepts of conjugated, isolated, and accumulated dienes, focusing on 1,3-butadiene as a key example. The video details how at low temperatures, the kinetic product forms rapidly, leading to a 1,2 addition, while at high temperatures, the thermodynamic product, a 1,4 addition, predominates due to stability. The summary also covers the stability of alkenes and the mechanism of the reaction, emphasizing the role of carbocation stability and the proximity effect in determining the major products.

Takeaways

• 🔬 Dienes are alkenes with two double bonds, and they can be conjugated, isolated, or accumulated based on the arrangement of these bonds.
• 🌟 Conjugated dienes are more stable due to resonance, which allows for the delocalization of electrons across the alternating double and single bonds.
• 🧪 1,3-Butadiene is an example of a conjugated diene, with double bonds on carbons 1 and 3, making it reactive in a unique way compared to isolated or accumulated dienes.
• ❄️ At low temperatures (e.g., -40°C), the kinetic product of the reaction between 1,3-butadiene and HBr is formed, which is the 1,2-addition product due to the faster reaction rate.
• 🔥 At high temperatures (e.g., 60°C), the thermodynamic product, the 1,4-addition product, is favored as it represents the most stable alkene configuration.
• 🏷️ The kinetic product forms faster at low temperatures and is represented by a single arrow in the reaction mechanism, indicating an irreversible process.
• ♻️ The thermodynamic product is formed at high temperatures through a reversible process, represented by two arrows with the rightward arrow being larger to show the favored direction.
• 📉 Alkene stability increases with the number of alkyl groups (R groups) attached to the carbons involved in the double bond, with tetrasubstituted being the most stable and monosubstituted the least.
• 🛠️ The mechanism of the reaction between 1,3-butadiene and HBr at low temperatures involves the diene acting as a nucleophile and HBr as an electrophile, leading to the formation of a secondary allylic carbocation.
• 🔑 The proximity effect and the stability of the carbocation intermediate are key factors in determining the major product at low temperatures, favoring the 1,2-addition (kinetic product).
• 🔄 At high temperatures, the reaction is reversible, and the most stable alkene product, the 1,4-addition (thermodynamic product), is formed due to considerations of alkene stability.

Q & A

What is a diene and how does it differ from a typical alkene?

A diene is a type of hydrocarbon that has two double bonds, unlike a typical alkene which has only one. The presence of two double bonds in dienes allows for additional types of reactions and properties compared to alkenes.

What are the different types of dienes mentioned in the script?

The script mentions three types of dienes: conjugated dienes, isolated dienes, and accumulated dienes. Conjugated dienes have alternating double and single bonds, isolated dienes have double bonds that are far apart, and accumulated dienes have double bonds that are very close to each other.

Why are conjugated dienes more stable than isolated or accumulated dienes?

Conjugated dienes are more stable due to resonance, which allows for the delocalization of electrons across the double bonds, making the molecule more stable than isolated or accumulated dienes where the double bonds are either too far apart or too close to each other.

What is 1,3-butadiene and how is it named?

1,3-butadiene is a specific type of diene with four carbons and double bonds located at the first and third carbon atoms. It is named based on the number of carbons and the positions of the double bonds, hence '1,3' indicates the positions of the double bonds in relation to the four carbon chain.

What are the two different products formed when 1,3-butadiene reacts with hydrobromic acid (HBr) under different conditions?

When 1,3-butadiene reacts with HBr at low temperatures, the kinetic product, which is the 1,2 addition product, is formed. At high temperatures, the thermodynamic product, the 1,4 addition product, is formed due to its greater stability.

What is the significance of the kinetic product in the reaction of 1,3-butadiene with HBr at low temperatures?

The kinetic product is significant because it forms faster at low temperatures. It is the product of an irreversible reaction and is represented by a single arrow in the reaction mechanism, indicating that it is the major product under kinetic control.

Why is the 1,4 addition product considered the thermodynamic product?

The 1,4 addition product is considered the thermodynamic product because it is the most stable product and forms as the major product at high temperatures. It is the result of a reversible reaction and is represented by two arrows in the reaction mechanism, with the one pointing to the right being larger, indicating its stability.

What factors determine the stability of the alkene products in the reaction of 1,3-butadiene with HBr?

The stability of the alkene products is determined by the number of alkyl groups (R groups) attached to the carbon atoms involved in the double bond. The more R groups, the more stable the alkene. Additionally, the position of the double bond and the type of substitution (cis or trans) also affect stability.

What is the role of the proximity effect in the formation of the 1,2 addition product?

The proximity effect plays a crucial role in the formation of the 1,2 addition product. It refers to the tendency of the bromide ion to react with the carbocation that is closer to it, which in this case is the secondary carbocation formed when hydrogen adds to carbon 1.

How does the mechanism of the reaction between 1,3-butadiene and HBr differ at low and high temperatures?

At low temperatures, the reaction is under kinetic control, favoring the formation of the kinetic product (1,2 addition product) because it forms faster. The reaction is irreversible. At high temperatures, the reaction is reversible and under thermodynamic control, leading to the formation of the thermodynamic product (1,4 addition product), which is the most stable alkene.

What is the significance of the carbocation stability in the reaction mechanism?

Carbocation stability is significant in the reaction mechanism because a more stable carbocation intermediate forms faster and requires less energy. In the case of 1,3-butadiene reacting with HBr, the secondary allylic carbocation is more stable than the primary allylic carbocation, influencing the direction of the reaction.

Outlines

00:00

🧪 Chemistry of Dienes and Their Reactions

This paragraph introduces the concept of dienes, which are hydrocarbons with two double bonds. It distinguishes between conjugated, isolated, and accumulated dienes, highlighting the stability and reactivity differences due to their structure. The focus is on conjugated dienes, particularly 1,3-butadiene, and how it reacts with hydrobromic acid (HBr) under different temperature conditions. At low temperatures, the kinetic product, a 1,2 addition product, is formed due to the faster reaction at carbon 1. At high temperatures, the thermodynamic product, a 1,4 addition product, is favored due to its greater stability as a disubstituted alkene. The paragraph also explains the concepts of kinetic and thermodynamic control in reactions, and the stability of alkenes based on the number of substituent groups.

05:00

🔍 Mechanism of 1,3-Butadiene Reaction with HBr

This section delves into the detailed mechanism of the reaction between 1,3-butadiene and HBr at low temperatures, leading to the kinetic product. It describes the nucleophilic attack of the diene's double bond on the electrophilic hydrogen of HBr, resulting in the formation of a carbocation intermediate. The choice of hydrogen addition to carbon 1 over carbon 2 is justified by the stability of the secondary allylic carbocation formed, which is more stable than a primary carbocation. The proximity effect and the stability of the secondary carbocation intermediate are the driving forces for the formation of the 1,2 addition product. The paragraph also contrasts this with the reversible reaction at high temperatures, where the most stable alkene product, the thermodynamic product, is formed due to the greater stability of disubstituted alkenes.

10:01

🌡️ Temperature Effects on Dienes Reactions

The final paragraph discusses the effect of temperature on the reaction between 1,3-butadiene and HBr. At high temperatures, the reaction is reversible, and the thermodynamic product, which features the most stable alkene, becomes the major product. The mechanism for the formation of the 1,4 addition product is outlined, involving the movement of the double bond to create a primary allylic carbocation, which then reacts with the bromide ion. The stability of the resulting alkene, influenced by the number of substituent groups, is the key factor in determining the thermodynamic product. This paragraph reinforces the importance of alkene stability in product formation and the role of temperature in controlling reaction pathways.

Keywords

💡Dienes

Dienes are hydrocarbons that contain two carbon-carbon double bonds. They are a key concept in the video as they are the main reactants in the chemical reactions discussed. The script distinguishes between different types of dienes such as conjugated, isolated, and accumulated dienes, with conjugated dienes being highlighted for their special characteristics due to alternating double and single bonds. The video uses 1,3-butadiene as an example of a conjugated diene.

💡Conjugated Diene

A conjugated diene is a specific type of diene where the two double bonds are separated by a single bond, creating an alternating pattern of double and single bonds. This arrangement allows for resonance stabilization, making conjugated dienes more stable than their isolated or accumulated counterparts. The video emphasizes the stability of conjugated dienes and uses 1,3-butadiene to illustrate this concept.

💡Isolated Diene

An isolated diene is characterized by double bonds that are far apart from each other, typically separated by more than one single bond. The script mentions that isolated dienes react in the same way as regular alkenes because the distance between the double bonds prevents any interaction between them.

💡Accumulated Diene

Accumulated dienes have their double bonds very close to each other, often adjacent or with only a methyl group in between. While the script does not provide a detailed explanation of accumulated dienes, it implies that they are less common or less stable compared to conjugated dienes.

💡1,3-Butadiene

1,3-Butadiene is a specific example of a conjugated diene mentioned in the script. It has four carbon atoms with double bonds between the first and second, and third and fourth carbons. The video uses 1,3-butadiene to discuss the reactions of conjugated dienes with hydrobromic acid (HBr) under different conditions.

💡Kinetic Product

The kinetic product is the major product formed in a reaction at low temperatures, where the reaction is under kinetic control. The video explains that at low temperatures, the reaction of 1,3-butadiene with HBr yields a 1,2 addition product, which is the kinetic product because it forms faster due to the proximity effect and the formation of a more stable secondary carbocation.

💡Thermodynamic Product

The thermodynamic product is the most stable product and is the major product at high temperatures, where the reaction is under thermodynamic control. The video describes the 1,4 addition product as the thermodynamic product, which forms when 1,3-butadiene reacts with HBr at high temperatures, due to the greater stability of the resulting disubstituted alkene.

💡Hydrobromic Acid (HBr)

Hydrobromic acid is the electrophile used in the reactions discussed in the video. It reacts with 1,3-butadiene to form either the kinetic or thermodynamic product depending on the temperature. The video script explains that at low temperatures, HBr reacts to form a 1,2 addition product (kinetic product), while at high temperatures, it forms a 1,4 addition product (thermodynamic product).

💡Carbocation

A carbocation is an intermediate in the reaction mechanism where a carbon atom has a positive charge. The video explains that the stability of the carbocation intermediate influences the reaction outcome. A secondary allylic carbocation, formed when hydrogen adds to carbon 1 in 1,3-butadiene, is more stable and forms faster, leading to the kinetic product.

💡Proximity Effect

The proximity effect refers to the influence of the spatial arrangement of reactants on the reaction outcome. In the context of the video, the bromide ion (from HBr) is more likely to interact with the secondary carbocation that is closer to it, leading to the formation of the 1,2 addition product at low temperatures.

💡Alkene Stability

Alkene stability is a concept that relates to the relative stability of alkenes based on the number and type of substituents attached to the double-bonded carbons. The video script explains that disubstituted alkenes are more stable than monosubstituted alkenes, which is why the 1,4 addition product is the thermodynamic product at high temperatures.

Highlights

Dienes are alkenes with two double bonds.

Conjugated dienes have alternating double and single bonds.

Conjugated dienes are more stable due to resonance.

Isolated dienes react similarly to regular alkenes.

1,3-Butadiene is a common diene with double bonds on carbons 1 and 3.

Reaction of 1,3-butadiene with HBr at low temperature yields the kinetic product.

Kinetic product forms faster and is a 1,2 addition product.

Reaction of 1,3-butadiene with HBr at high temperature yields the thermodynamic product.

Thermodynamic product is the most stable alkene product.

Alkene stability is determined by the number of R groups attached.

Tetrasubstituted alkenes are more stable than tri-, di-, or mono-substituted alkenes.

Trans alkenes are more stable than cis alkenes.

The kinetic product is formed under irreversible reaction conditions.

The thermodynamic product is formed under reversible reaction conditions.

Diene acts as a nucleophile and HBr as an electrophile in the reaction mechanism.

Hydrogen adds to carbon 1 to form a more stable secondary carbocation.

Proximity effect and carbocation stability drive the formation of the 1,2 kinetic product.

Resonance structures explain the formation of the 1,4 thermodynamic product.

Alkene stability is the driving force for the formation of the 1,4 thermodynamic product.

Transcripts

in this video we're going to talk about

the 1 2 and the 1 4 addition products

with dienes

but let's begin our discussion with

dienes

so here is a typical alkene

it has one double bond

a diene

is basically an alkene but with two

double bonds

now there's different types of dienes

this particular diene

is

known as a conjugated

diene

the reason why it's conjugated is you

have alternating double and single bonds

here it's a double bond this is a

carbon-carbon single bond and that's a

double bond

conjugated dienes have special

characteristics

now there are other type of dienes that

you need to be familiar with

so this

is an isolated diene

because the double bonds are

too far apart from each other

and then

you also have accumulated dienes

so that's where the double bonds are

very close to each other

of these three the most stable is the

conjugated ion

due to resonance

an isolated diene reacts in the same way

as a regular alkene because the double

bonds are far apart

but a conjugated diene it reacts in a

different way and we're going to focus

on that in our discussion today

so let's begin

so let's start with this common diene

this is called one three butadiene

if we count the number of carbons

we have a total of four carbons so think

of butane

and we have a double bond on carbon one

and three so it's called one three

buta diene

now what we're gonna do is we're gonna

react this dyeing

with hydrobromic acid

hbr

but we're going to react it under

two different conditions

we're going to react it at low

temperature let's say

negative 40 degrees celsius

and at high temperature which we'll say

at 60 degrees celsius

what will be the major product

for this reaction what would you say

well at low temperature

you're going to get something called the

kinetic product

the hydrogen is going to add on carbon

1.

this is let's call this carbon one two

three and four

by the way due to the symmetry of this

diene these two alkenes are equally

reactive

so the hydrogen is going to go on carbon

one

the bromine is going to go on carbon two

we're going to have a double bond

between three and four

so this is called the one two addition

product

because hbr added

to carbon to one and two

now this is also called the kinetic

product

because it forms faster

now at high temperatures

the thermodynamic product will be the

most stable product it's going to be the

major product

the hydrogen is going to add to carbon 1

but the bromine atom is going to add to

carbon 4. and we're going to have the

double bond between carbons 2 and three

so this is called

the one four

addition product

as you can see

on carbon one we have the hydrogen and

on the carbon four we have the bromine

atom

now this is also called the thermo

dynamic product

it is the product with the most stable

alkane

i mean the most stable alkene i take

that back

so this is the product that forms at

high temperature

now

the kinetic product

the reaction associated with the kinetic

product is an irreversible reaction so

we're going to use one arrow to

represent it

the reaction associated with the

thermodynamic product is reversible so

we're going to have two arrows but

typically the one pointing to the right

is going to be the bigger

arrow because

this product

is the most stable product

now let's talk about why this product is

the most stable product

the answer has to do with the alkene

stability

notice the number of r groups that are

attached to this alkene

this particular alkene has two r groups

so it's a disubstituted alkene

this alkene

only has one r group

that are attached to the two

carbon atoms that are double bonded so

this is a mono-substituted alkane

so it is the least stable of the two

so make sure you understand that

difference the kinetic product is the

product that forms faster ideally at low

temperatures the thermodynamic product

is the most stable alkene product that

forms as the major product at high

temperatures

now let's briefly review

alkene

stability so this is

a tetrasubstituted alkene

it has four

alkyl groups attached

to the two carbon atoms that are double

bonded

this is a tri-substituted alkane

it has three r groups

a tri-substituted alkene is more stable

than

a di-substituted alkane

here we have a trans

dye substitute alkene

and that's usually more stable

than a cis disubstituted alkene

and here we have a mono substitute

alkane

which is more stable

than ethene which i'm just going to

write like that

so the more r groups that you have

around an alkene the more stable it is

and that's going to help you to identify

the thermodynamic product

but now let's go back to the reaction

of one three butadiene with hbr

and this time let's talk about the

mechanism

so let's react one three butadiene with

hbr at low temperature conditions

so as was mentioned before we're going

to get the kinetic product as the major

product even though both products can

form

this reaction is under kinetic control

so the one that forms faster is going to

be the major product when the

temperature is low

now in this reaction

the diene is going to behave as the

nucleophile

hbr the acid is going to be the

electrophile

so we're going to write up a mechanism

and to show the arrow

just remember

when when drawing the arrow it's going

to start from

a region of high electron density to a

region of low electron density

this double bond is electron rich

hydrogen being partially positive is

electron poor so the arrow

basically describes the direction of

electron

flow so to speak

so when the double bond

interacts with the hydrogen

the bond between hbr is going to break

those two electrons

is going to be pulled towards the more

electronegative bromine atom

now here's a question for you

why should we put the hydrogen on carbon

1 and not on carbon 2

notice what happens if we put the

hydrogen on carbon 2.

if we were to do that we would get a

primary

carbocation

but if we were to put the hydrogen on

carbon 1

the plus charge is going to be on carbon

2.

so we get a more stable secondary

carbocation but notice that it's

adjacent to a double bond

so what we get is a secondary allylic

carbocation and so that's the reason why

hydrogen is going to add on carbon 1.

it's because we get a more stable

carbocation intermediate

so that's the first thing you need to

consider

where will the hydrogen go

now what will the bromine atom do

or more specifically the bromide ion

now by the way

this structure has a resonance structure

we could move the alkene here if we take

those two pi electrons

and move it to the left

we can get another intermediate so this

is going to be

a less stable primary allylic

carbocation

so this is secondary

and this is primary

so just looking at the carbocation

stabilities

this particular resonance structure is

more favorable than this one

so because the secondary allylic

carbocation is more stable it's going to

form faster

because there's less energy that's

required

to generate this particular resonance

structure

now

which carbocation will the bromide ion

interact with

is it going to be the secondary

carbocation or the primary allylic

carbocation

well as was mentioned before the fact

that this is more stable

means that bromide is more likely to

interact with this secondary carbocation

but that is not the only reason there's

also something called

the proximity effect

because bromide is so much closer to the

secondary

carbocation it's easier for it to

interact with that particular

carbocation

this positive charge is further away

from the bromide ion so it's less likely

that the bromide ion is going to

interact with the primary

carbocation so therefore there are two

factors

that are favoring the one-two kinetic

product

the first most important factor is the

proximity effect

and for this specific example the second

thing is

we have a more stable carbocation

intermediate

so once the bromide ion

combined with the carbocation

we're going to get

the one two

addition product

so at low temperature conditions

this is going to be the major product

and the driving force is the proximity

effect and the fact that we get a more

stable carbocation intermediate

now let's draw the resonance structure

for the formation of the 1 4 product

so let's react 1 3 butadiene

with hydrobromic acid

at high temperature conditions so let's

use positive 80 degrees celsius

so at a high temperature this reaction

is reversible

so the most stable alkene product will

be the major product

so like before

we're going to react the alkene with hbr

and we're going to put the hydrogen on

the primary

carbon

so that we can put a positive charge on

the secondary carbon

now to get the 1 4 product we need to

draw the resonance structure

where this double bond is going to move

here

and now we have the plus charge

on the primary allylic carbocation

and then the bromide ion

is simply going to

interact with the carbocation

and so that's how we can get

the 1 4

thermodynamic product

so that's the mechanism

that you can write to show it

and so

for this one the driving force is alkene

stability

the thermodynamic product is the one

with the most stable alkene product