Chemistry Chapter 6: Substituted Alkanes

6.2 Nomenclature of Substituted Alkanes

Define Alkyl halide

Compounds of the formula CnH2n+1X, where x is F, Cl, Br, or I

What are haloforms

CH3X

What are vinyl halides

H2C--CHX

What are allyl halides

H2C--CH-CH2X

What's important to note about primary, secondary, and tertiary carbons

It's important to note that primary, secondary, and tertiary carbons are all sp3 hybridized

We never refer to a vinyl carbon, a carbon part of a double bond, as being primary, secondary, or tertiary

How do we name alcohol

Find the longest carbon chain that includes the carbon attached to the alcohol

Drop the "e" in the parent name and adding the suffix "ol"

Giving the OH group the lowest possible number because ti has priority

Labeling all other substituents in the prefix alphabetically

Allyl alcohol

trans-Crotyl alcohol

Benzyl Alcohol

Phenol

Amyl Alcohol

ethylene glycol

Glycerol

What are molecules containing 2 OH groups called

Molecules containing two OH groups are called diols, but there is an often used common name, glycol

note that in the naming of molecules with two OH groups the final "e" of the alkane parent compound is not dropped

What are amines

Amines are derivative of ammonia, NH3

Successive replacement of a hydrogen on ammonia leads to primary, secondary, and tertiaryamines(NH2R, NHR2, NR3)

What are ammonium ion

Quaternary nitrogen compounds that are positively charged are called ammonium ions R4N+

How are primary amines named?

Primary amines are commonly named by using the name of the substituent R, and appending the suffix "amine"

How are secondary and tertiary amines in which the R groups are the same named?

dialkylamines, trialkylamines

Amines in which the two R substituents are different have the substituents named in alphabetical order

What is the IUPAC convention for naming amines

Find the longest carbon chain with the carbon attached to nitrogen and use that parent hydrocarbon name

Replace the "e" at the end of the parent carbon chain with "amine"

Number the chain such that the amine has the lowest possible number

Label all substituents in the prefix in alphabetical order

Secondary and tertiary amines will have the alkyl group listed in alphabetical order in the prefix as N-alkyl or N,N dialkyl if the groups are the same

What if a molecule contains both an Amine and an alcohol group

The IUPAC system has the alcohol group with a higher priority than the amine group

So if the molecule contains both an amine and and an alcohol will be named as the alcohol, with the amine listed as amino in the prefix

How are cyclic amino compounds named?

cyclic amino compounds are widely found in nature and are almost invariably known by there common names

These compounds are often named as "aza" analogues of an all carbon system

How are ammonium ions named

Ammonium ions are named by alphabetically attaching the substituents in the prefix

Don't forget to include the name of the negatively charged counter ion

How are ethers named in the common scheme and IUPAC scheme

Ethers are compounds that have the form R-O-R or R-O-R'

In the common scheme, the two alkyl group attached to the oxygen are listed in alphabetical order and the word ether is appended

In the IUPAC system, ethers are named as Alkoxy (RO) alkanes, and groups are listed in the prefix in alphabetical order

An ether is a subordinate group, which means in the IUPAC system it can only be named as a prefix

How are cyclc ethers named

Cyclic ethers are named as either oxa ring compounds or as oxides by counting the number of methylene groups within the ring

So a cyclopentane with an oxygen atom in the ring can become either oxacyclopentane or tetramethylene oxide

How do we name epoxides?

Three-membered rings have their own naming schemes due to the frequency at which they are encountered

The most versatile IUPAC method has you treat the epoxide as a substituent. It has you fin dthe longest carbon chain and then use the term epoxy in the prefix for the two carbons attached to the ring oxygen

6.3 Structure of substituted alkanes

Describe the structure of alkyl halides

Simple alkyl halides are nearly tetrahedral, which means the carbon that the halogen is attached to is approximately sp3 hybridized

The bond in alkyl halides involved an overlap between a carbon's sp3 orbital and the orbital of the halogen whose principle quantum number ranges from n=2 to n=5

The c-x bond length increases and bond strength decreases as we read down the periodic table

Describe the bond angles in alcohols

Alcohols are derivative of water, and thus they somewhat resemble water in structure.

The bond angle is expanded a bit from 104.5 degrees in H-O-H to 108.9 degrees in simple alcohols R-O-H

The O-H bond length in alcohols is very little changed from the O-H bond in water.

The C-O bond in alcohols is shorter and stronger than the C-C bond in ethane

Describe the orbital interaction diagram for the formation of the carbon-oxygen in methyl alcohol

Notice how an electron in an orbital near the electronegative oxygen atom is more stable than it would be in an orbital near carbon

Describe the bond lengths in amines

In simple amines, the N-H bond length is shorter than the C-C bond length in alkanes

SImple amines are hybridized approximately sp3 at the nitrogen and are thus pyramidal. The bond angles are close to 109.5 but cannot be exactly the tetrahedral angle

Describe the bon dangles in amines

In tertiary amines where all three substituents are the same, the R-N-R bond angles are the same

But for less symmetrical amines, all three R-N-R angles cannot be the same

Describe amine inversion

The most important structural feature of amines comes from the lone pair of nonbonding electrons acting as the fourth subtituent on the pyramid

The pyramid as a result can undergo an "umbrella flip" amine inversion, forming a new mirror image pyramid

The nitrogen is also capable of being a stereogenic center

Define Amine inversion

The conversion of one pyramidal form of an amine into the other through a planar, sp2-hybridized transition state

Describe the energy barrier to amine inversion

The barrier to inversion in amines is very low, only about 5-6kcal/mol, and isolation of one isomer, even at low temperature, is very difficult

Inversion is too facile, rapid inversion interconverts enantiomeric amines

Describe the bond length and angles in ether

The C-O bond length in ethers is similar to the C-O bond length in alcohols, and the opening of the angle from H-O-H(104.5) to R-O-H(109) to R-O-R(112) continues.

Because the bond angles in ehter is 112, we can say that the oxygen is approximately sp3 hybridized

6.4 Properties of Substituted Alkanes

Describe the polarity of substituted alkanes

Substituted alkanes are polar molecules. halogens are relatively electronegative atoms and strongly attract the electrons in the carbon-halogen bond of alkyl halides, which have a dipole moment of approximately 2D

Describe the polarity of alcohols

In alcohols, the presence of the electronegative oxygen atom ensures that bonds are strongly polarized and that a substantial dipole moment exist

Because of this, alcohols are strongly associated through dipole-dipole interactions and through extensive hydrogen bonding

Define hydrogen bonding

A low-energy bond between a pari of electrons on an electronegative atom such as oxygen, nitrogen, or flourine to a hydrogen on a oxygen, nitrogen or fluorine

Describe hydrogen bonding in terms of alcohols

Hydrogen bonding interactions can be quite strong, 5kcal/mol

This has the effect of increasing the boiling point of alcohols far above that of alkanes

Describe hydrogen bonding in terms of amines

Amines, like alcohols, have higher boiling point than hydrocarbons ofsimilar molecular weigh tdue to hydrogen bonding

Though this effect is not as great as in alvohols due to the reduced electronegativity of nitrogen

Describe the bp of tertiary amines

In tertiary amines, hydrogen bonding cannot occur, and thus the boiling point decreases

Describe the polarity of ethers

Ethers are polar, but only for the smallest classes of ethers do polarity effect physical properties

For example, diethyl ether has nearly the same boiling point as pentane and is only modestly soluble in water

Are alkyl halideseither basic or acidic?

Alkyl halides are neither acids nor bases.

They cannot donate a proton and have no basic lone electron pairs

Are alcohol and amines acidic or basic?

Alcohol and amines, can be proton donors, and because of their lone electron pairs, can be bases as well

Derive the equilibrium constant expression for the dissociation of an acid in water

relate pka values to the strength of an acid

The stronger the acid, the lower the pka value

any compound with a pka lower than 5 is regarded as a reasonably strong acid

any compound with a pka below 0 is regarded as a very strong acid

Describe protonation of an alcohol

Protonation converts an alcohol into an intermediate oxonium ion.

Without protonation, breaking the R-OH bond would be very difficult, because both positive R+ and negative OH- ions would have to be formed. Thus we saythat hydroxide is a poor leaving group and alcohols do not ionize easily.

After protonation however, the leavingn group is no longer hydroxide, but water, formation of the neutral molecule wawter is a much easier process

Describe the strength of protonated alcohols

When an alcohol is protonated it turns into an intermediate oxonium ion, ROH2+. This makes the oxygen of the alcohol carry a positive charge. The consequence of this protonation is that the alcohol becomes a much stronger acid

What is the challenge with protonating alcohols?

The problem with protonating alcohols is that it requires a very strong acid, this is because alcohols are not naturally very strong proton donors or acceptors.

Yet once the alcohol is protonated, the oxonium ions are much more effective at donatinh protons

Describe the pka values of oxonium ions formed from alcohols

oxonium ions formed from alcohols have 'pka values in the -2 range

What does bronsted acidity depend on?

bronsted acidity depends on the hydroxyl hydrogen. Loss of this hydrogen to a proton acceptor, a bronsted base, leads to the conjugate base of the alcohol, called the alkoxide ion. (RO-)

Define alkoxide ion

the conjugate base of an alcohol, RO-

Compare the acidity of alcohols to water

alcohols are about as acidic as water

How come the more alkyl groups on the alcohol, the weaker acid it is?

This is largerly due to the inductive effect of alkyl groups and the stabilization of the conjugate base

Alkyl groups donate electrons through what's called an inductive effect, and as a result, electron density is pushed towards the moreo electronegative oxygen atom. This serves to make the OH bond less polar, and make the alcohol less likely to donate a proton

When an alcohol loses a proton, an alkoxide ion is formed, the stabilization of this conjugate base of the alcohol is crucial. Alkyl groups destabilize the negative charge on the oxygen because they donate electron density to the oxygen atom, this serves to make the conjguate base of the alcohol less stable, meaning the alcohol is less likely to donate a proton in the first place

Why is the explanation that alkyl groups are electron donating not correct?

Research has shown that the order of acidity of a list of alcohols was reversed in the gas phase. Thus the intrinsic acidity of the alcohol was opposite to that foun din solution

The acidity order measured in the solvent represent a powerful effect of the solvent, not the alcohol itself

Describe the stability of organic ion

Organic ions are almost all unstable species, and the formation of the alkoxide anion depends critically on how easy it is to stabilize them through interaction with solvent molecules, called solvation

Describe the difference in acidity between tert-Butyl alcohol and Methyl alcohol in solution

tert-Butyl alcohol is a weaker acid than methyl alcohol in solution due to the large tert-Butyl alkoxide ion is difficult to solvate.

The more alkyl groups, the more difficult it is for stabilizing solvent molecule to approach

describe the acidity of alcohols in the gas phase

In the gase phase, the intrinsic properties of the alcohols themselves determine their acidity. In this phase, alkyl groups serve to stabilize the alkoxide ion by delocalizing elewctron density through a process called hyperconjugation

Define hyperconjugation

The interaction of electrons in ansp3 hybridized orbital with an empty orbital

Describe the basic properties of amines

Like water and alcohols, amines are Bronsted bases.

Ammonia, primary, secondary, tertiary amines are all weak bases

Protonation of an amine leads to the formation of the ammonium ion

This is important in biological chemistry, when biological reactions neeed a base, it is often amine that plays that role

Describe how we can measure the basicity of amines

If the ammonium ion is a strong acid, the related amne must be a weak base.

That means, the higher the pka value of the acid, the stronger the conjugate base will be

Why does methylamine's ammonium ion have a higher pka than ammonium?

Recall that the more substituted a carbocation ion is, the more stable it becomes.

Similarly, the more substituted an ammonium ion, the more stable it is and the less likely it is to give up a proton, resulting in a higher pka value

Why does trimethylammonium have a lower pka than dimethyl ammonium?

In the liquid phase, this graph doesn't seem to make sense, how can a more substituted ammonium ion be less stable?

Again we look to the gase phase and see a trend that we would expect, the more substituted the ammonium ion, the higher its pka

So now we can assume that there must be some efffect present in solution that is not present in the gas phase

How are ions in solution stabilized?

By either electrostatic stabilization or through hydrogen bonding

What are the two effects that alkyl groups have on the stability of the ammonium ion

The alkyl group helps to stabilize the ammonium ion by dispersing the charge

it destabilizes the ion by interfering with solvation by making intermolecular solvation more difficult

Describe the effect of adding additional methyl groups on the acidity of the ammonium ion

Little steric interference with solvation occurs when we replace one hydrogen with a methyl group in ammonium, and further more there is stabilization, so pKa increases by a full unit

Replacing a second hydrogen with a methyl group yields a balance between steric hindrance of solvation and stabilization of the ammonium ion, the result is no net change in the pKa

Replacing a third hydrogen with a methyl group now results in the steric hindrance of solvation being far greater than the stabilization derived from a more substituted nitrogen atom

Describe the effect of adding additional methyl groups on the acidity of the ammonium ion in the gas phase

In the gas phase, solvation does not occur, and the ammonium ion only derives the stabilization from being more substituted, so only the stabilizing effects remain

Describe the value of pkb in relation to base strength

the lower the pkb, the stronger the base

Amines have a pkb of around 4, which makes them quite strong b ases compared to other organic molecules

Compare the basicity of amines and alcohols

Amines are much more stronger bases than alcohols are, it is much harder to remove a proton from ammonium that it is from hydronium

Why is the oxonium ion more acidic than the ammonium ion

oxygen is a more electronegative atom than nitrogen and thus bears the positive charge less well.

Discuss the removal of a proton from a primary or secondary amin

Removal of a proton from an amine yields an amide ion

An amide ion is less stable than an alkoxide ion due to the negative charge being borne on the less electronegative nitrogen atom

Describe the overall acidity and basicity of amines and amide ions

amines are good bases but relatively poor acids

The conjugate bases of amines, the amid ion, or an even stronger base than the amine itself

What are the factors the determine the acidity of a compound?

ISHARE

Induction

Size

Hybridization

Aromaticity

Resonance

Electronegativity

How the inductive effect on acidity

Inductive effects are transmitted through sigma bonds

Any bond between two different atoms are polarized towards the more electronegative pair

Explain the difference in acidity between trichloroethanol and ethanol

Trichloroethanol has a pka nearly 4 units below that of ethanol.

The key here is to look at the conjugate base following the deprotonation of the two alcohols. If we look at trichloroethanol, the C-Cl bonds stabilize the nearby negative charge on the oxygen atom by withdrawing electrons through the sigma bond framework

Anything that stabilizes a conjugate base will make it easier to remove that hydrogen to get that conjugate base

Explain the effect of size on acidity

Generally the large the atom, the better it is at bearing the negative charge.

The larger nucleus, with more positively charged protons, stabilizes a negative charge better than a smaller nucleus

Explain the difference in acidity between HF and HI

Furthermore, a small hydrogen makes a strong bond with a small flourine atom and a weak bond with a larger iodine atom due to very poor orbital overlap

Thus HF is a weaker acid than HI by a very large amount

This is a comparison within the same column of the periodic table

Explain the effects of hybridization on acidity

The more S character in an orbital, the lower in energy an electron in it is.

Thus an electron in an sp orbital is lower in energy than an electron in an sp2 orbital, which is lower in energy than an electron in an sp3 orbital

As a result, acetylenes are far stronger acids than alkenes, which are far stronger acids than alkanes

Explain the effect of resonance on acidity

Non solvated charged atoms are typically higher in energy than neutral atoms, which means that they are harder to form.

As a result, it is especially difficult to form ions in which the negative charge is localized on one atom

As a result, a molecule that gives a delocalized anion upon deprotonation is going to be a stronger acid than a molecule than give a localized anion upon deprotonation

Describe the effect of electronegativity on acidiy

All else equal, it is easier to form an anion in which the negative charge is on the more electronegative atom than it is to form an anion where the charge is on the less electronegative atom

Thus HF is a much stronger acid than R-OH

6.5 Solubility

How does hydrogen bonding occur

For hydrogen bonding to occur, there neweds to be a proton donor(NH2 or OH) and a proton acceptor(a lone pair of electrons)

What are protic solvents?

Solvents that can donate a proton are called protic solvents

Protic sovlents are typically polar and have high dielectric constants

What is a dielectric constant

a measure of the ability of a solvent to separate charges

What are aprotic solvents

Solvents that are not proton donors

How effective are protic solvents in dissolving polar molecules

Polar solvents are particularly good at dissolving molecules capable of hydrogen bonding

Formation of hydrogen bonds within solution is highly stabilizing

How effective are aprotic polar solvents at dissolving other polar molecules

Polar aprotic solvents can help disperse the partial charges in other aprotic polar molecules and thus dissolve them well

This occurs through alignment of opposite charges

6.6 Formation of Substituted Alkanes

6.7 A reaction of alkyl halids" Synthesis of Alkanes

Why are alkyl halides important in synthetic chemistry?

Alkyl halides are crucial in synthetic chemistry because they allow the formation of C-H and C-C bonds

What is a organometallic reagent?

A molecule that contains both carbon and a metel

Usually the carbon is at least partially covalently bonded to the metal

What happenswhen an alkyl halide is added to a cold mixture of either magnesium or lithium maetal and an ether?

When an alkyl halide is added to a cold mixture of either magnesium of lithium metal and an ether solvent(R-O-R solvent, typically diethyl ether or THF), the metal begins to dissolve in an exothermic reaction

The end result fo the reaction is either a grignard reagent or an organolithium reagent

What is a Grignard reagent?

A strongly basic organometallic reagent formed from a halide and magnesium in an ether solvent

The simplest formulation is RMgX

What is an organolitihum reagent

A strongly basic organometallic reagent, R-Li, formed from a halide and lithium

What is the reaction mechanism for Grignard reagent synthesis

What is the reaction mechanism for organolithium reagent synthesis

Describe the equilibrium of the Grignard reagent

The Grignard reagent is in equilibrium with a mixture of magnesium halide and dialkylmagnesium compounds. The Grignard reagent also incorporates two molecules of solvent

What ever the structure, grignard reagents are highly polar molecules that are very strong lewis bases

describe the radical-transfer reaction of the machanism fo the formation of a Grignard reagent