Intro to Hydrocarbons - Online Tutor, Practice Problems & Exam Prep

Hydrocarbons, the simplest organic compounds, consist of carbon and hydrogen atoms. Alkanes, with single carbon-carbon bonds, follow the general formula:

$C_n{H}_{2n+2}$

indicating a saturated structure with the maximum number of hydrogen atoms. Alkenes and alkynes, containing double and triple bonds respectively, have formulas:

$C_n{H}_{2n}$

and

$C_n{H}_{2n-2}$

showcasing unsaturated structures with fewer hydrogens due to the presence of π bonds. Cycloalkanes, which are ring-structured alkanes, share the

$C_n{H}_{2n}$

formula with alkenes, while aromatic hydrocarbons like benzene have a unique

$C_n{H}_n$

formula. The spatial orientation of hydrocarbons is influenced by bond rotation; alkanes exhibit free rotation around single bonds, whereas alkenes and alkynes lack this due to the rigidity of π bonds, leading to different compounds based on the spatial arrangement of substituents.

Intro To Hydrocarbons

Now we'll call that hydrocarbons are the simplest organic compounds composed solely of carbons and hydrogens. Here in this list, we're going to go over different types of hydrocarbons. So if we start with the first row, we have alkanes. Alkanes are just carbons that are single bonded to one another. Now if we pay attention, remember each one of these bonds connects 2 carbons together. Carbon must make four bonds. If we look at the two carbons in the middle, they each have two hydrogens.

Now focus on either one of these carbons and you would see that they are connected to 4 surrounding atoms, meaning that their hybridization is SP3, their generic formula. If we were to look here, we'd have 4 carbons and you would have 10 hydrogens. And that's because generic formula for an alkanis CnH2n+2, or North is the number of carbon atoms. So if we said that we need an alkane that has five carbons, what is its molecular formula? It'd be C5H2∗5+2. That'd be C5H12.

Now alkene is when you have two carbons double bonded to each other. They still need to make four bonds. They're making three, so they each have one hydrogen. That's invisible. Notice that the number of hydrogens decrease as we added that double bond between the carbons. These carbon C are connected to 123 surrounding atoms, so they are SP2 In terms of hybridization, each of these double bonded carbons lost a hydrogen, so we lost two hydrogens overall. So our generic formula has changed to CnH2n. We've dropped the +2 because we've lost those two hydrogens.

Alkynes are triple bonded carbons, so there's one here and here. They're making four bonds right now, so they don't need any extra hydrogens on them. These two SPS carbons are SP hybridized because they're connected to two surrounding atoms. And think about it, we just lost another two hydrogen. So the generic formula becomes CnH2n-2. Cycloalkanes are just alkanes that are in rings. If we sold, they're all still single bodied. In this ring we have carbons all around. Each of these carbons has two hydrogens on them, right?

So if we think about it, focus on one of them. Let's focus on this one right here. It is connected to 1234 surrounding atoms, so it is SP3 hybridized here. If we looked, we have 5 carbons for 10 hydrogen. The ratio is there's double the amount of hydrogen to carbon, so that means a formula will be CnH2n. So a cycloalkane has the same generic formula as an alkene. We had to lose 2 hydrogens in order to enclose this alkane into a ring.

Finally, aromatic aromatic is really referring to a benzene ring. A benzene ring is 6 carbons in a ring with alternating double bonds. Each of these carbons has one hydrogen. So a benzene has a formula of C6H6. Another way of showing it is like this. This is changing it into more of a skeletal formula. Each of those carbons are double bonded. They're each connected to three surrounding atoms, so they are SP2 hybridized. And here it's a one to one relationship between carbon and hydrogen. Since it's C6H6 O, the generic formula is CnHn, right?

So these encompass all the different types of hydrocarbons that you may encounter when dealing with deeper types of ideas and organic chemistry. But remember, essentially they're all just compounds with carbons and hydrogens here. Some of them have single bonds, some of them have double bonds, some of them have triple bonds or are in rings OK, but they're all hydrocarbons at the end of the day.

Intro To Hydrocarbons Example

So in this example question it says classify each of the following hydrocarbons as an alkane, alkene or alkyne. So remember an alkane is when we have carbons just single bonded to one another. If we take a look at the options, remember every line connects 2 carbons to each other. In this option B all of them are single bonded to one another. So they're all alkanes.

Well, it is an alkane. This is the only alkane that we have and alkene is when we have carbons double bonded to one another. These two carbons here are double bonded to each other, and as long as two carbons are double bonded to each other, it is an alkene. These two carbons are double bonded to each other, so this is also an alkene.

An alkyne is when we have two carbons triple bonded to one another. We have a carbon here and a carbon here, so option C would represent an alkyne. So this is how we describe our options ABC and D from the following example question.

Saturated and Unsaturated Hydrocarbons

Now with hydrocarbons, we now talk about saturated versus unsaturated. Now a saturated hydrocarbon means that all the bonds are single bonds, so all the carbons connected to each other are single bonds. This allows each carbon to have a Max possible number of hydrogen atoms. So if we take a look here at this image, the one on the left is a saturated hydrocarbon. Those 3 carbons have to make four bonds, and to do this we'd have to add hydrogen to them.

An unsaturated unsaturated would mean that we have at least one double bond or triple bond, double or triple bond meaning that we won't be able to have a maximum number of hydrogens. So if we take a look here, we have this double bond here. What effect will that have? Well, the carbon on the left is single bonded, so it still has its maximum number of three hydrogens. But because of that double bond there, carbon again remember, needs to make four bonds. This carbon here is already making 123 bonds, so it only needs one hydrogen.

This carbon here is making 123 bonds, so it also needs one hydrogen here and one hydrogen here. What's the difference? Well, these two carbons, this one here had two hydrogens. But what? The inclusion of a double bond not only has one this carbon here has three hydrogens, but the inclusion of the double bond means now it has only two. So adding up π bond, adding a double bond means that we're going to have to forfeit one hydrogen atom per carbon.

And if we were to make a triple bond again, we'd have to forfeit one hydrogen atom between those carbons. OK, so this is the difference between saturated where there are no double bonds or triple bonds. So you have the maximum of hydrogen atoms on carbons and an unsaturated hydrocarbon, where now double bonds and triple bonds decrease the total number of each atoms possible.

Intro To Hydrocarbons Example

Here in this example question, it says classify the following hydrocarbons as saturated or unsaturated. Remember, a saturated hydrocarbon possesses only single bonds between its carbons. In this image, A&C would represent saturated hydrocarbons.

Every line connects carbons together. All these carbons are single bonded to one another, meaning they'll have a maximum number of hydrogen atoms attached to each of them. Unsaturated would be these two unsaturated hydrocarbons have a double bond or a triple bond present.

This decreases the maximum number of H atoms that can connect to a potential carbon. Remember, carbon must make four bonds. Adding those double bonds decreases that number, right? So A and C would be saturated. B&D would be unsaturated.

Write the molecular formula for an alkane with 5 C atoms.

• A. $C_5H_{12}$
• B. $C_5H_{10}$
• C. $C_5H_{14}$
• D. $C_5H_8$

Write the molecular formula for an alkyne with 4 C atoms.

• A. C₄H₈
• B. C₄H₁₀
• C. C₄H₆
• D. C₄H₄

Which of the following molecular formulas might indicate an alkene?

• A. C₇H₁₆
• B. C₆H₁₂
• C. C₅H₈
• D. C₄H₁₀

Bond Rotation and Spatial Orientation

So in our continued discussion of hydrocarbons, we're now going to take a look at bond rotation and spatial orientation. Here we're going to say carbon. Carbon bonds and alkanes can rotate freely, and that's because of the presence of a single bond. So here this is a carbon and this is a carbon and we rotate around a single bond. Now by rotating that, we're going to keep one side stationary, meaning we're not going to move it, we're going to keep this side still.

So that matches U with this side, and we're going to rotate the other part, the right side when I'm rotating so this red ball is up here, so when I rotate it'll rotate down to where the blue is. O there it goes right there. And then when I rotate that down, this blue comes to where this part is. So there goes the blue part right there. And then that green thing has to shift up to this to where the red was. So there's the green part right up here.

How would this look like in terms of a skeletal formula? So again, there's free rotation around the single bond. So we rotate, we keep this side stationary so it stays where it is. Everything else is spinning. This OH would rotate to down to where the down to where the H is. So that's why it's here. This H would rotate to where this methyl group is, and then this chapter three group here would rotate up here.

Now if we have a double bond which is in alkenes, we cannot rotate. There's no free rotation around it. A double bond. This leads to two different spatial orientations and therefore two different compounds. These two carbons are double bonded, so we can't rotate. That means that these two Blues are locked on the same side and these two greens are locked on the same side.

So it would not be equal to this molecule here, where the Blues are opposites of each other and the greens are on opposite sides of each other. If we show this as a skeletal formula, these two HS are locked on the same side, but here they're on opposite sides. There's no free rotation, so these will represent two different compounds. Just keep in mind when we're dealing with double bonds and also triple bonds, there's no free rotation around that bond. Things are locked where they are.

Intro To Hydrocarbons Example

Here it says determine if the two structures below are the same or different compounds. Well, both compounds have two carbons double bonded to each other. Remember, there is no free rotation around that double bond, so things are stuck where they are.

The one on the left has an H and an F next to each other, but the one on the right has 2HS instead next to each other. The one on the left has a BR and an H next to each other, but this one here has a BR and an F next to each other.

We have different groups next to one another in both molecules or both compounds, and we can't rotate around that double bond so that they're the same. Because of this, they would represent different compounds O. Here our answer would be option B.

Which of the following is not a valid bond rotation?

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

How can hydrocarbons be identified?

Hydrocarbons, which are compounds composed exclusively of hydrogen and carbon, can be identified using several methods depending on the complexity of the molecule and the level of detail required.

1. Physical Properties: Initial identification can often be made by examining physical properties such as boiling point, melting point, solubility, and density.
2. Combustion Test: A simple test involves burning the substance. Hydrocarbons typically burn with a blue, non-sooty flame. However, if the combustion produces a sooty flame, it indicates the presence of a higher proportion of carbon, suggesting a more complex hydrocarbon structure.
3. Infrared Spectroscopy (IR): This technique identifies hydrocarbons by their characteristic absorption of infrared light. Different functional groups within the hydrocarbon will absorb at specific wavelengths, creating a spectrum that can be used to identify the compound.
4. Nuclear Magnetic Resonance (NMR) Spectroscopy: NMR can provide detailed information about the structure of hydrocarbons, including the environment of the hydrogen atoms and the carbon framework.
5. Mass Spectrometry (MS): MS can be used to determine the molecular weight of the hydrocarbon and to infer structural information from the pattern of fragmentation during the ionization process.
6. Gas Chromatography (GC): For mixture analysis, GC can separate the components of a hydrocarbon mixture and provide information about the individual compounds present.

How can unsaturated hydrocarbons be identified?

Unsaturated hydrocarbons can be identified through a few different methods, the most common of which is the use of chemical tests. One such test is the bromine test. In this test, bromine water (a solution of bromine in water, which is typically brownish-red in color) is added to the hydrocarbon. If the hydrocarbon is unsaturated, it will contain double or triple bonds, and the bromine will react with these bonds, causing the solution to decolorize. This indicates the presence of unsaturation as the bromine adds across the double or triple bonds.

Another method is the Baeyer test, where a solution of potassium permanganate (KMnO₄), which is purple, is used. If unsaturation is present, the solution will become colorless or brown as the permanganate reacts with the double or triple bonds.

Infrared spectroscopy is also a technique used to identify unsaturated hydrocarbons. The presence of characteristic absorption bands in the infrared region can indicate the presence of C=C (alkenes) or C≡C (alkynes) bonds typical of unsaturated hydrocarbons.

Which of the following is a property of a hydrocarbon chain?

A hydrocarbon chain is a series of connected carbon atoms with hydrogen atoms attached to them. One of the fundamental properties of a hydrocarbon chain is its hydrophobicity, meaning that it is nonpolar and does not mix well with water. This is because the bonds between the carbon and hydrogen atoms are relatively nonpolar, making the overall molecule lack the ability to form strong interactions with polar molecules like water.

Additionally, the length and shape of the hydrocarbon chain can affect its properties. For instance, longer chains generally have higher boiling and melting points due to increased van der Waals forces. The structure can be either branched or unbranched (straight-chain), and this can also influence the physical properties of the molecule. Saturated hydrocarbon chains, with single bonds between the carbons, tend to be more rigid, while unsaturated chains, containing one or more double bonds, have kinks that make them more flexible. These structural variations make hydrocarbons versatile in their applications, ranging from fuels to the creation of different types of plastics and synthetic materials.

Which formula represents an unsaturated hydrocarbon?

An unsaturated hydrocarbon is a hydrocarbon that contains at least one double or triple bond between carbon atoms. These bonds create a deficiency in the number of hydrogen atoms attached to the carbon chain compared to saturated hydrocarbons, which only have single bonds and the maximum number of hydrogen atoms.

The general formula for an alkene, which is an unsaturated hydrocarbon with at least one double bond, is:

_CnH2n

For example, ethene (ethylene), with the molecular formula:

_C2H4

is the simplest alkene.

Alkynes are another type of unsaturated hydrocarbon that contain at least one triple bond and have the general formula:

_CnH2n-2

An example is ethyne (acetylene), with the molecular formula:

_C2H2

Remember, the presence of double or triple bonds is what characterizes unsaturated hydrocarbons, and these bonds can be identified in the structural formula of the compound.